M4A1 MK3 doesn't have any mods available,
FAMAS MK3 has a dot sight and a trigger to make it full auto, since it is 3 round burst by default,
Para usually is used with a barrel, grip, and stock.
XM8 just has 2 different triggers. (i think)
I think R4 Rem just has 3 different barrels.
I'm not exactly sure how repair costs work, but capsule guns cost more to repair. The repair cost might depend on the price of the gun.
For sniper rifles, usually you want to go for the most damage and range possible, and if it has a mod to increase the magazine, that is good too. For example, the FRF2 can be used without any mods, but it can also have long range barrel and larger mag, so it 1 hit kills more at range and has 10 round mag instead of 5.
Also, Galil MAR doesn't have any mods, but costs more to start with.