Depending on how greatthe body, interior and suspension really is will be the answer... Suspension being the big question, as once a few things need to be replaced, youre spending over a few thousand on it.
If anything but the engine and drivetrain were shot, Id say to keep it. Rubber bushings on a commuter car wont leave you stranded in west philly at 2am... but an bum engine running poorly might eventually foul up plugs or whatnot so bad that its over.
If you dont have a friend or a good deal to putin the work, youre looking at $$$. Even if you get a free engine and free replacement parts to make it really good, youre still looking at $$$, and to some extent, uncertainty, especially if its not a rebuild.
If its a car that costs you >$30k to replace with its modern equivalent, Id say go for it. Id have no issue replacing the engine or drivetrain in my 300D (228k) or 318i (110k) at any point (though both are still like new fortunately) - though some may say that even that is silly. However, for an econo car that can be had either identical in age for next to nothing, or newer for not that much, or brand new for less than $20k, heck, probably less than $15k if speced out right, I dont think there is a reason to keep a car that can be replaced with a similar economy car that will produce less exhaust emissions, be better from a safety and technology standpoint, have more power and potentially better fuel economy as well.
That said, depending on how you would plan to dispose of it, I would probably keep feeding it 15w40 and a stop leak/burn product until you extract more utlity from it... You just got a new clutch? Wel, drive it until the tires wear out, or there is something else major... Youll get even less for the shell (or you might just be plannng to roll it to the junkyard anyway), but youll put off your purchase and extract more use from the vehicle, so youre better off, IMO,
JMH