My father in law has an old overhead projector, basically I wanted to take out the default projection lens and replace with a triplet. I also thought about gutting the default lighting system and installing a 400w halide in its place. How feasible would this be?

I also started out with an old 3M OHP. I found there can be a lot of good quality high-performance parts to use. But if you have a 15" LCD, then the OHP components really are a little "too small". They are usually designed to illuminate sheets of transparency paper or about 11" square. A 15" LCD screen is 12 inches wide by 9" tall, so it really doesn't fit within the OHP operating envelope.

That's not to say however you still can't make use of some of the OHP parts. I was able to get the light engine to work by moving it farther back from the rear fresnel and using a longer focal length fresnel (317mm versus 220mm). This allowed the light cone to expand more before it reached the fresnel and fully illuminate the LCD.

I also was able to use the varifocal triplet lens and mirror assembly from the OHP, but I had to accept some slight distortion at the very corners of the image that makes it seem slightly "rounded" overall, almost like an old TV screen. And I can't get a uniform focus from corner to center of the image.

So to answer your question - Yes, you might be able to use an OHP projector depending on the size of your LCD and your tolerance for imperfections. Because there will be some inevitable compromises you will have to make...

You might be best off building a new enclosure for your PJ, because you will likely run out of space for your LCD parts and fresnels and light engine. Plus you will want to minimze the unwanted light leakage and "pollution". An OHP lets a lot of light escape between the fresnel and projection lens.