Stan, I've gone through your pain many times in the past. In fact, my first bike almost wasn't, due to near buying a SC for my 94' Lightning…I was on the phone ready to order and bailed out. Best decision ever. I'd have lost a boat load on the deal and not started making money in bikes.
You won't make an S-runner anything else than a glorified run-around truck. Toyota didn't put the best power plant in it for upgrades and it really is a warmed over Taco when you look at most of the mods. Any money you put in won't just be lost, but you'll also further depreciate the value of the truck. The market doesn't like modified vehicles, especially SC'd, etc.
Sell it. Buy a Dodge SRT-10, if you need extra cab/ 4 door. Similarly, you also could get a Ford Harley Davidson (same package as Lightning, 4 door). BOTH of those trucks are increasing in value as of late (low miles ones). Lighting are incredible for the dollar…as fast as your M5 and carry a full load almost. I loved all the ones I had. The SRT-10 won't ever be seen again, IMO. It'll be worth good dollars later. The reports that it doesn't handle well are more aimed at the 4 door version; the 2 door version is a good little handler. Despite that, the 4-door handles really well for a large, heavy truck. If you spend the money on a good clean one with lower miles (get it fully loaded if you get the 4-door) and keep it in good shape, it'll have a good chance to increase in value as well as be something pretty rare. An S-runner isn't something car guys covet.
You're smart with your money. Don't put one dime into the S-runner. Change horses now and enjoy something new. The S-runner won't be near SRT-10/ Lightning quick with that SC.