A plastic (acrylic) tank is fine. And the heater isn't an issue. (All of my tanks are acrylic, and have heaters. Also, I wash them weekly with hot water.)
Heaters only take the water temp up to about 82F. But our bodies are at 98.6F! So if a heater isn't even are warm as we are!
(That said, you don't want to use a heater when it's not submerged, because in that case, the temp *could* rise to dangerous levels. As long as it's used underwater though, it should only heat the water to the desired temp of roughly 78-82F.)
If your tank hasn't been used in several months, and has been dry this whole time, anything bacterial or viral (unless it's mycobacterial) is likely dead. However, just in case.....
To disinfect your old tank, hot water followed by air drying (especially in sunshine) is probably OK. If you want to step it up a level, use vinegar, then rinse really well and air dry (in the sun, if possible). And if you are really concerned, you can wash it out with a dilute bleach solution, then rinse, rinse, rinse, rinse, air dry, and double dose on water conditioner the first time you refill.
(I had a fish die of unknown illness. I used dilute bleach to clean his acrylic (plastic) tank, rinsed LOTS and LOTS with hot water, then air dried for several days, and used extra water conditioner when I refilled. I've had a new fish in there for a few months now, and he's fine.)
Last edited by LittleBlueFishlets; 02-24-2013 at 06:10 PM.