if anything i think $5k is a little underestimated. depending on the quality of your setup, you can easily spend:
$300 on tailshaft
$10-1500 on brakes
$200 for reconditioned brake booster
$50-2000 for a diff
$30-1000 on seats alone, you want to slide around on 70's vinyl?
$0-300 for a steering wheel,
$1500 for an aftermarket ECU
$300-1000 for a set of useful gauges
$300-500 on fuel setup.
$150-300 for thermofan
$50-1000 for some nice extractors
$300 for high flow cat
$500-1500 for engineers report
$300 for blue slip/pink slip/change of details/registration
haven't included all the other little things that can seriously add up or the RWD water setup, hyrdraulic master cylinder/lines, or timing belt/waterpump service you should do, or motor seals, radiator, shocks/springs, H4 conversion, mufflers, wheels/tyres
all this and you will either want to spend another shitload to make it handle good or put a turbo on it for some real fun.