To focus well the answer to which warfare can come to Syria from their allies (without count Russia, Iran has a key role) and the black markets in the continental Asia, The money is a key factor. Everyone with a minimun knowledge knows the specifications of every piece of warfare. More the current owners. Then obviously, the most useful warfare will have higher price than less effective warfare, but still useful (when the other option is to use civil vehicles or to use only portable/man portable weapons). The higher prize of the most useful warfare is justified because of the lower incentive of the current owners to sale them.
The money is a bareer to see better warfare in Syria, and the lowest technological level has a welcome in this war. To provide material for large military forces in war is not easy and cheap, and even $5K by unit of T-55 ($1M by 200 without ammunition) can become too much in adition to other needs of ammunition and material. Syria today is not in position of selecting material, Syria depends of the external gifts/aid and of cheapest material available of Russian design, that for sure has been receiving it the last years (the numbers for the evolution of the equipment of their Armed Forces suggest it). Today Syria must fight adapting themselves to what they have available, and to what the current owners of these weapons want to sale.
To remember the list including the MiG-19 and the ASU-85:
This is is what I would expect to be supplied to Syria from third countries by their suppliers. It would be available at low price.
Heavy Towed Weapons:
023mm ZU-23-2 (also from Russia)
082mm Vasilek (also from Russia)
Mobile Land Warfare:
MiG-19 (J-6, F-6, Q-5, A-5, FT-6,...)
To have more modern warfare retired by Russia like Su-7, FROG-7, T-54, T-62, T-55, MiG-21, T-64 or to have warfare present in the Russian Armed Forces, surely Syria would need to pay significantly more.
From what I see in the public data, the number of units of the warfare cited in the box, available in the continental Asia (except Russia), is not big today, In most of the cases less than three hundreds.
In the refered to Russia, I think Syria did some contracts that finished its purchasing capability. Today the relation would be more about external aid, than about purchases of warfare. In this context, we can understand the direct military operation, and also the transfers of material that we can expect today. To remind it:
This is what I would expect to see moved to Syria from the Russian Armed Forces
023mm 1960 ZU-23-2 (Until to be totally exhausted in the Russian Armed Forces).
082mm 1970 Vasilek (Until to be totally exhausted in the Russian Armed Forces).
Iveco LMV (Until to be totally exhausted in the Russian Armed Forces. Surely continue present in other non-military Russian security forces).
152mm 1975 Giatsint-B
120mm 1986 Nona-K
122mm 1960 D-30
100mm 1961 (M)T-12
152mm 1987 Msta-B
And this is what I would expect to see moved to Syria from other Russian security forces
122mm 1960 D-30 (Until to be totally exhausted in the Russian security forces).
MI-24/25/35 (Until to be totally exhausted in the Russian security forces).
Ka-27/28/29/31/32/35 (Until to be totally exhausted in the Russian security forces).
The main reasons to include the Su-25 and the helicopters would be that Russia may want to tranfer to Syria the riskiest operations of air support and the transfer of this warfare would have very low affect in the capabilities of the Russian Aerospace Forces (the helicopters even are not part of their warfare).