I can't answer all your questions, but here's the things I can think of:
1. The pump needs to handle the "lift" from the bottom of the sump to the top of the tank. How many liters/gallons per hour you want is pretty much depending on how much water movement you want in the tank. 2-5 times the tank volume is a good starting point, I would think (so ~ 200-500 gph).
2. You need suitable hose to go from the sump to the top of the tank, with either a 90-180 degree bend, and perhaps a nother 90 degree bend if you want a spray-bar.
I'm assuming the drilled tank is fitted with a suitable overflow drain, which should eliminate the need for any hoses for draining. If that's not the case, then I don't know what you need, because it will depend on the actual setup.
If you heat in the sump, you shouldn't need a second heater in the tank, the water will need a lot of (iow LONG) contact with cooler air to make it noticably cooler.
Obvious question: Doesn't the current owner have some idea of what is needed? If the current owner hasn't had the tank up and running, is there a manufacturer or previous owner to ask? If you can't trace the source of the tank, it MAY not be such a bargain, because you may end up with a lot of hard to solve problems. I'm speaking from experience here where I bought a "bargain" tank with a few bits missing, and ended up saving a whole 11p (that's about 20 US cent) after I managed to find a web-site that sold the parts I needed for 15-20% less than my LFS... Nice tank, but it would have been a lot less hassle to just buy the kit with everything in it... The glass in the tank isn't very expensive, all the other bits tend to be...