better to costomise an a5 than buy an x7

It depends, calculate the cost of what you want to turn an A5 into vs the X7 with appropriate parts.

I prefer the A-5. It's less bulky, with no real difference in weight. The iron
sights aren't completely worthless like the X-7's. Also, It's cheaper, and has
more mods (functionally speaking. eg low pressure kits and expansion
chambers.)

The only place the X-7 wins, really, is the looks department. You can make
it look like whatever you want, but all you've really done (usually) is change
the way it looks. It's still the same marker, only now it's got on a prettier
shroud.

For speed, A5 with E-grip, Qeph, jet clicks, and squishy paddles.

 

http://cgi.ebay.com/ws/eBayISAPI.dll?
ViewItem&ssPageName=STRK:MEWAX:IT&item=390017244444

best deal i've found.

I also like the 14" or 16" Progressive barrel on this PMI HPA tank to boot.
4500/68ci usually turns up the best price/capacity listings. some kind of
aftermarket hopper would be better than the stock one, too...