You will do well to start talking with Montenegro about the mutual benefits of such a possibility.
The purpose of such arrangements is that they are rock solid reliable places to send ships that wont suddenly become hostile overnight.
Personally I think you interest in SAMs is well founded, but your plans to simply buy from the shelf are a little short sighted.
I think your angle should be getting together with the Russians to develop new SAM systems that would be relatively cheap but also capable, able to reach high up into the air so air defences cannot be overflown, but also a mix of capable but also cheap to use in numbers.
I am thinking of an entire air defence network designed to be simple and capable that can make a NATO type attack expensive for the attacker.
There will not be S-500s or even S-400s in this network because such installations are too hard to hide. You want elements that can operate 24/7 in all weathers that don't rely on large easy to target radars. Passive sensors, IR and electronic, difficult to jam or target with anti radiation equipment.
Long thin multi stage SAMs that can reach long distances and can be deployed in a way to not look like SAMs... a row of single launchers along a street to look like telephone poles... but all linked to a non centralised AD network based on the internet but kept seperate so that if one part goes down it doesn't take the rest with it.
For smaller missiles you want something like Pantsyr that can fire on the move and is compact enough to be a very difficult target.
You guys know what you did to evade NATO airpower... just add passive sensors and missiles that can reach high and far that are as easy to move and hide as possible.
You performance would have gotten an A+ if you had just shot down so many aircraft they couldn't hide it.
Another factor too is that a lot of the aerial targets you will want to deal with will be UAVs and UCAVs which are small elusive but not hard to intercept or hard to bring down targets. Most of them will lack situational awareness and will keep straight and level so simple and cheap missiles could be used.
The US is developing a swarm type UCAV that can be launched in large numbers, something like the laser guided 57mm gun fired missiles the Russians are working on would be a great way to deal with such threats as their available munitions capacity would be high and their rate of fire would be high and jamming them would be tricky. Yet they would be relatively cheap and mobile.
A larger calibre longer range model in 125mm calibre might be useful for towed gun positions firing at aerial targets as an example too.
The point is too that the missiles you develop with the Russians on this basis will not only be useful for Russias traditional clients who often face NATO or US aggression, but also for Russia herself to fill gaps with capable but cheaper mobile easy to hide and move weapons.
Russia have backed out of T-95 development because their focus is on mobility... the Tunguska weighs about 34 tons so there is plenty of room for weight reduction.
Modifications of the BTR series to allow a rear troop compartment and rear ramp access should also make possible a variant with the engine and driver in the front plus a small turret in the centre with small phased array radars and thermal sighting systems and the rear can hold a large pallet of vertical launch missiles like TOR and later Morfei perhaps. (The latter is a new fire and forget lock on after launch IIR guided missile based on a new AAM for the PAKFA).
Or engine at the front and turret in the middle with 100 rounds of 57mm anti aircraft laser guided shells with another 2-3 reloads in the rear of the vehicle would be ideal for dealing with UCAV swarms which will effectively blind the enemy which relies on such recon means.