I've been taking the same approach for years. Buying a car towards the end of its depreciation curve and maintaining it well leaves you with a manageable reliable *car*. not an asset. when people are saying "it's not worth it" when faced with a large repair bill against the notional value of their car, they're confusing it as 'putting money into an asset'. Cars - on the whole - are liabilities, not assets!
As with Andy's approach above, the $1000 chunks works for me. I've been spending about $1k/year on my 855-T5 for the past 8 years. I paid around $9k for it, it's worth about... well, whatever anyone would pay for it. It's reliable and fun to drive, and I've learned plenty working on it. Paid for some jobs, done others myself with my mate on his driveway. One year it's not mechanical but turns out to be a set of tyres and an alignment. It's a grand a year ongoing, basically.
You're lucky, the car has cost you nothing. As Andy illustrates above, you've got nothing to lose in catching up on that deferred maintenance - if it's a solid car to start with. Perhaps spending a case of beer with Andy looking it over and drawing up your battle plan will help you establish your budget. If you're importing parts yourself, I use FCPEuro.com as the prices and quality is good, along with the service. You might be able to carve up purchases into orders that squeeze in under the GST/Duty thresholds as well, with careful purchasing.
Do your basics, get the engine maintenance stuff addressed. Belts, filters, fluids (cooling, engine oil, diff oil, power steering fluid)... mounts, cooling (pump, hoses). all those gaskets you mentioned - with a mate, and taking your time, it's labour intensive though not rocket science. Do it with a mate, break down the jobs into 'work packages' so you're not overwhelmed by the enormity of the task, build your confidence as you go, enjoy a couple of beers with each job. Does the timing chain and guides need replacing - check against the specs. now the trans - the service is easy. Buy your ATF in a 20 litre pack; it's way cheaper that way. At this point you should have a car that's relatively sorted for reliability from the basics. (hopefully your heater core and valves are okay).
Next you're into stop and go. How are the brakes? How is she steering and handling?
You're probably sold on driving and owning a quality german engineered car over a japanese or korean beater. Do remember - nothing is free - if you play, you pay. you may be $1000 in (and $150 in beer tax) by the time you get to the end of the trans service... what else would you be driving for $1200?
As Andy says, it's not necessarily the *right* way to do it; it's one way of doing it that works for some of us. YMMV.