Yes, there were ISU-122's that used the same barrel as the IS-2. Just add the barrel and you are good. The real vehicles were made to be able to switch out the 122 and even 152 main guns if needed, although I do not think it was a easy conversion...
The Zvezda kit is rougher in detail and has very poor tracks but is more dimensionally accurate out of the box. The DML kit has better starting detail and very nice tracks, but suffers from too-small roadwheels and a "squished" hull. You need to add about 2mm all around the lip of the lower hull to get the upper hull high enough to look right. If you do not care so much about accuracy, either kit is ok OOB.
BMP-2 is more of a problem. DML's kit is the only one out there with just a lot of reboxes having been done (as stated above). To do one correctly, it needs:
New upper hull or reworked upper rear hatched (they shoudl not cant) - SP Desings
New turret - Model Point or SP Designs.
New gun barrel - Model Point
New Smoke Grenade Launchers - Model Point
New Tracks - Master Club including new sprockets and roadwheels
The newer Eduard PE set for the BMP-2 is also a must.
As you can see, you almost build a whole new kit just fixing up the DML one. But it would be a stunner when done.
(For a cheaper alternative for a Russian UN vehicle, I would recomend the Eastern Express/Alanger BMD-D.)
