Welcome to the FNZAS forums.
Filter, for Oscars it's going to need to be a big one. Canister filter will be best, perhaps a sump if you feel up to it.
Heater, the general rule is 1 watt per litre. 340L tank = 340 watts. Standard heater sizes are 25w, 50w, 100w, 200w, 300w. Perhaps two 200 watt heaters and place them at opposite ends of the tank?
Lighting, considering it's an Oscar tank, lighting should be simple due to lack of plants. You can slap on pretty much any Fluoro tube as long as it looks good . Being a DIY'er I'd stay away from the lighting setups from petstores as they're quite expensive. Hardware and lighting stores are often much cheaper and wholesalers on TradeMe are even better.
Good luck