That filter should be ok for the tank. You've got a flow rate of about 4.5-5 times per hour so it really depends on what you want with regards to a powerhead. If you're going planted and are going to get loads more plants then you could get something with a flow of around 1500lph to give you 10x turnover for your tank which is generally accepted as ideal for a planted tank.
However if you're just doing it because of the little warning light on your heater I wouldn't worry. Maybe the bubble wall moving has messed some internal calibration on the heater and if you shut it off for 5 minutes (the heater) and put it back on it would recalibrate?
Powerhead wise you could look on ebay for a Hydor wavemaker. I've got an APS
2500lph one or you could go for the
2000lph one maybe. You'd have a turnover of about 13x which would be ok.
Hydor do a 900lph or 1600lph I think so if you can find them fairly cheaply on eBay you could do a lot worse. Here are some
brand new ones but you may be able to get them cheaper if you hunt around or are happy to get a second hand one. (mine is)