Good idea or waste of $$
Here
I have a hard believing that 500 miles a year lead to your tranny failure. My guess would be the transgo kit is not installed correct or your line pressure is incorrect.
If you were going to skip the nitrous, I would do a conservative build. The stock 4l60 will live pretty long in a car with bolt-ons and a 3500 stall, so long as you have a good cooler. Good clutches and steels is all you probably need. If you are going to spray more than a 100 shot or spray the 100 shot regularly, a built tranny is in the cards.
I don't know that I would buy from a shop in not located within driving distance unless they had a solid reputation. The warranty is worthless because shipping costs and downtime will mean that you'll never send the tranny back to them if you have a problem. The fact that they have a good e-bay rating doesn't mean their tranny's are good, only that they ship their parts fast. I'm not saying the tranny isn't good; only that e-bay feedback isn't a great measure for a mechanical device like a tranny with a 12 month warranty.

