IMO, formulate some sort of gameplan. For the moment, forget about the mileage of the engine.
From what I can gather from your post, it sounds like you do plan on modding the car. SO here's what I would do (mine is just about to hit 200k and I bought it with 168k):
First, I would make a list of all the things I would want to do- be it short-term or long-term. At least, you'll have a set goal, at least as far as performance goes, and you could look up information on all the parts.
Secondly, I would save some money for emergencies. In this case, it would be in preparation for another engine, should it go sometime down the road.
Once you have banked enough for an engine and labor costs (unless you plan on doing it yourself), save up/get some mods. If you plan on running with the GA still, the mods can be trasferred over to the next engine.
Now as far as which engine to use, do some research

Alot of people talk about doing the SR swap, but if you pockets aren't deep enough, you might want to postpone that idea. There's alot of parts that you'll have to aquire just to get the car in running condition not to mention the cost of getting it in tip-top shape is the most intimidating part. (looking at probably 3k for all the parts, then another 1-3k on labor if you're not doing it yourself.) You could turbo the GA for less than the SR swap, but the only downside would be the current limitation of power (a Turbo SR will blow you out water considering their aftermarket).
All in all, just do the basics. They aren't crazy expensive, and you could still save for another engine at the same time.