The other rifles can be bought when you have money to burn. It's best to wait until later (lvl 20+), and then go with the Rahim and later on the MK18. Sniping generates the least amount of $/xp by design. This is vital to making the most money/xp over time.

Don't 'rent' equipment unless absolutely necessary. It will be your friend for quite a while. If you are at level 10, you can get the Katiba.