A new TPS from Toyota is around £100 + VAT. A second hand one will always be a gamble. I wouldn't be replacing parts until you know which part is at fault. (You could spend alot of money and still have the same problem). Have you checked for air leaks on the intake manifold, etc? How's you oil level? As for lights not coming up on the dash, bulbs could have died or been removed to hide faults. I would personally disconnect the battery to clear all codes the reconnect it and go for a long drive so the ecu can relearn its info and gain any fresh fault codes.