A couple of things to consider...
1) Have you seriously taken a look into what it cost to own a Porsche? Like, spent time on the forums, etc? Later model year porsches are prone to stupidity like door locking mechanisms that fail around 60k that then throw CEL's "because-German", failed sensors, clutch accumulators that go out every <10k miles, tail lights crack by design and need replaced, etc etc. What I'm really saying is these aren't Hondas and they arent even problem prone FD's. The mechanicals of the Mezger are solid, most of the rest of the car is more problem prone than it should be. Most of those areas wouldnt change with an LS swap, you'd be replacing the one piece thats solid.
2) Have you taken a look into what it cost to MODIFY a Porsche? This is the killer if you are used to FD parts cost. "Cheap" coilovers are $2k, moderate $2800, high end as much as you want to spend. Clutches seem to start around 1500, brakes expensive, GT2 body mods seem to start around 3k for a bumper, etc. The aftermarket parts are priced very much relative to the $135k car is was new
I mention 1) and 2) above because I went in glossy eyed with blinders on. I can afford it because it honestly just means I pay normal people car maintenance cost (even though I am doing labor myself) but as my wife will attest I bitch about it a lot...
3) If you are still interested in a 911, seriously why not get a 996 Turbo for ~38-42k and call it a day? 996TT with 30-60k are available for high 30's low 40's with a little bit of looking. My DD is a 2001 996TT with ~60k miles on it. Its a pretty fun car, but overwhelmingly expensive to modify. Its *nearly* as fast as my 400rwhp LS1 FD, but AWD so gets off the line much easier. With a $900 Cobb tune it would likely be faster than my LS/FD in a straight line. Its certainly tons more comfortable for longer drives, and right now is quiet exhaust wise. (Its stock) My car friends tell me its "grown-up fast" compared to my FD which is obviously loud, looks "tuner-ish" and "juvenile", etc. (I say that with the utmost respect and admiration for my FD as I honestly like it BETTER than the 996TT, but the 996 is clearly the better driver and more "acceptable" at business meetings)
Just sayin, don't discount the engine already in the 911TT. The mezger is no slouch and (if you care to spend the money on clutches, turbos, etc) easy to get numbers north of 600AWHP
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Edit: Felt the unnecessary need to post a picture of the 996
