The cage rules changed for new logbooks issued in 2012. Although the new regs don't rule out bolted in/bolted together cages they affectively make it impossible to build one as they now require diagonal bars in the roof, hoop and door openings thus making welded together, welded in cages the way to go. The 2012 regs also mean cacges are substantialy heavier. A ready built, already logbooked car will be a much better starting point even if you have to go right through the car and refurbish everything. Peter Spight's car would be a decent car for the $$.
Normally I wouldn't reccomend starting in a WRX, but they're cheap enough thesedays and readily repairable so I guess if you can why the fuck not?