Hold off on buying anything until after cert. Then consider diving just a watch, depth gauge, and tables for a while. Once you're ready for a computer, whether it's in 5 dives or 50 dives, spend the few hundred bucks for a Petrel, which can be had for less than $850 if you find the right dealer. Trying to save $100 or $200 on this stuff is pennywise, pound foolish. Buy once, cry once.
If you're having trouble coming up with the extra cash, find some savings on the BCD by buying a used BP and wing, which will serve you well and only run about $300 or so for a great Halcyon or DSS used rig, less for something less up there in terms of brand name