Hi Brian,
I took a gamble and bought an old rough looking ultrasonic tank on e-bay(about 7 litre size tank), cost me £56 plus £12 postage and it works great but if it hadn’t I would have had the grief of sending it back etc. As to size it depends on what you are wanting to clean in it, a 2 litre tank might just be enough to cover a smiths mantle clock but nowhere near big enough for a long case mech. Worse case scenario, if you are not looking to do many big clocks then get a smaller one, there is a lot to be said for hand cleaning if you do have to clean something that wont fit in the tank. As for digital display and a heater etc, my tank has an on and off switch, it gets warm through use so I wouldn’t worry about a heated version, keep it simple, there is less to go wrong! For cleaning solution I use soft soap, ammonia and water. Oh and check the actual dimensions of the cleaning tank, 2 litres might sound ok but the shape of the tank could stop you being able to completely immerse the clock plates, which is not good.
Paul.
p.s. hopefully will get your tailstock done this Friday 
