Let me just go get my armchair general's ushanka
Good thing I played all those grand-strategy video-games
OK let's start with the geography before discussing anything else:
Scotland can be divided into two parts -
The lowland, more densely populated South
And the highland, sparsely populated North
Both are at least moderately forested; there is little in the way of plains and flat farmland. This gives the defender the advantage and even in our age - greatly improves the possibilities for concealment, guerrilla warfare, ambushes and defensive positions.
It also reduces the possibilities for maneuver and brings a penalty to any attacker wishing to employ tanks and mechanised infantry. Which again is a boon to the Scots; they can be expected to be heavily disadvantaged in such capabilities & vehicles compared to a potential adversary such as Great Britain or the Europeans.
An adversary attacking with ground forces from the South would face a quickly narrowing strip of land leading to the main population and urban centres of Scotland along the horizontal Glasgow-Edinburgh axis. From then on its urban warfare and moving past that - the highlands; which would pose even more problems for a conventional mechanised/armour assault.
However Scotland does have a weakness inherent in its geography too and a very serious one at that. Namely - the huge amount of amount of coastline that it has relative to its overall territory/area. By means of shipborne cruise missile attack, amphibious landings or quick commando raids it would be possible to assault a wide variety of Scottish military bases or facilities, cut supply-lines, encircle formations and generally wreck havoc on what would otherwise be a straightforward strategy of funneling enemy forces attacking from England into one big chokepoint
Scotland would be best-served by prioritising the defense of its coastline and national waters, as well as its airspace (vital to deny not only the enemy airforce but also naval cruise missile attacks). Money should be allocated for high-tech capabilities in these branches first and foremost, while the army can make do with less expensive gear; it needs no sophisticated equipment to wage a fighting retreat from one fortified defense line to another, and with the right use of ambushes and terrain; engagement distances can be made much shorter and even obsolete equipment can be as highly effective as anything else.
I envision a sort of vehicular guerrilla warfare on the part of Scotland's air, coastal defenses, mobile artillery units and small naval vessels that would be able to continue to inflict severe losses on enemy air, naval power and marines even when faced with heavy enemy superiority in air and naval forces (which is pretty much a given).
It's also a given that Scotland's air force, centralized air defense and coastal co-ordination will be suppressed and destroyed too sooner or later - which is why its important for the equipment used to be capable of operating completely autonomously if required.
So.. taking all this in mind:
- S-300/S-300PMU2; perfectly adequate, highly-mobile unit for the job of long-range air-defense and defeating cruise missile strikes, denying enemy air support. These units will however be the centre of attention and will eventually all be caught and destroyed. Employing these older models as opposed to the newer S-400s will enable money to be saved for an even more vital part of the air defense envelope..
- BUK-M3; the very latest variants. The most important parts of Scotland's air defense network, and crucially - the most survivable. Certainly the best ratio between survivability and price/capabilities. These critters proved themselves in the 2008 Georgian war; continuing to inflict casualties even in the face of Russian air superiority. Normally they work as a part of a unit, together with specialised vehicles - but if need be, they can scatter like ants to different locations or simply hide in the forest, mountains, etc... because each vehicle has its own radar and can operate autonomously. Coupled with ammunition& fuel resupply vehicles, they can continue operation for weeks after the air defense network has broken.
- Pantsir-S1; fulfills two major functions - SHORAD for Scottish airfields & other vital military facilities, and mobile air defense & ground fire support for army brigades. Should be acquired in relatively small numbers (its vital but expensive) in variants appropriate for both roles.
- Igla-S; MANPADs to fend off enemy helicopter assaults, CAS aircraft and destroy transport helicopters. Should be acquired in large numbers and issued extensively to ground forces, naval coastal defense forces, the territorial army and as part of base garrisons. Enemy air power and transportation can be expected to be extensive; and large amounts of man-portable AA missile systems can greatly increase the effectiveness of a guerrilla campaign if it comes down to it.
- SU-30MK2; optimised for the role of naval attack, but can be equipped as air superiority fighters in order to support the air-defenses, depending on the circumstances. But the main idea is that they would carry Scotland's supersonic anti-ship missiles and inflict heavy pain on any frigate-size or larger enemy naval vessels that have arrived to deliver precision strikes or support amphibious operations. The MiG-29/35s would be just as good for the purpose of air-superiority; taking into account Scotland's relative small size - but their range and radars would not be enough for the purpose of long-range anti-ship operations. Meanwhile the Su-35 only really has an advantage over the Su-30MK2 in terms of maneuverability; which is not vital for the role.
- SU-25 Frogfoot; in several modernized variants and outfitted for specific roles such as anti-tank. Su-25s will also serve gladly for the role of trainer via their 2-seat trainer variants (Su-25UBM), that can also be employed in battle as full-fledged Su-25s. Making a tactical reconnaissance variant/mod of a dozen in partnership with Rosoboronexport could be an option too; I know there was such a variant on the drawing board 20 years back. Yak-130s or L-139s would be nice of course as trainer and light-attack aircraft (albeit not CAS), but they're expensive and the money spent on them would be better spent on more Su-30MK2s or modernized Su-25s.
I'll finish the rest later.