Whole cheap guns aren't worth starting with if intend on building a FMU uber l33t sopmod mk whatever. You'll end up with 99% of the original rifle in the parts bin by the time you're done.
BUT...you do get the option of having something that does spit bbs out while you're buying/scrounging/waiting for that next part.
Buying a whole good FMU rifle to start with...is much the same as the above but you've just jumped $300-400 up in the start price (and consequently the cost of the parts that are rolling around in your spare parts bin).
BUT...the parts that you have left over have some resale value because they were decent to start with. Who wants a cheapo M4 barrel with a front triangle sight molded right into it? Nobody. Who wants a solid CA M15 outer barrel and front triangle sight? Lots of people.
BUT...if you intend on doing nothing to it (and about 90% of people start out thinking that...), buy a whole good FMU rifle. Get the internals looked over (unless you know what you're doing) and hit the fields.
BUT...most of the above applies to M4/M16 variants. If you're getting an AK, G36, MP5, PSG1, G3, etc... buy the best whole rifle you can (if the model you like has a metal body version get it). Then have the insides checked out and go have fun.
BUT...and this is most important....
If you can't afford it...don't do it. The rifle itself is just about a third or quarter of what you'll spend on other stuff. That stuff can be pieced together...but there are some things that you need right out the gate. Save up. Use the time to carefully pick out what you want (unless you want to end up with bins and bins of stuff you hardly use...
Best of luck,
But...custom builds are more fun!