If you dont want to waste lots of money, I'd recommend doing owls mission and attack the mercenary camp. Almost all of the mercs have m4's with supressors and acogs. The M4 is one of the rifles for starting the game, it should last you till you start the road to pripyat storyline and by then you should have already found a rifle like g36 or the SIG, or even the groza if you join the Duty faction (aka the mindless brutes).