kind of you to take him/her in. hopefully his little leg will start feeling better soon.
as for the set up-
reasonably big for now but you need a uva/uvb bulb along with the heat lamp-the turtles need it for their shell growth. also do you have a filter? turtles are very very very messy and create loads of waste. good filtration and good maintenance are necessary. it was difficult to tell but make sure the basking area is accessible and the turtle can completely haul themselves out and dry out their shells-dont want him getting shell rot-very difficult to treat and the healing time may take several months.
keep in mind that as small as this little guy seems, within a year's time it will be approx 4" or more in shell length and will need an upgrade. they also can live over 35 years-that is a big commitment. where are you located? if appropriate, i would suggest nursing him back to health and letting him grow until he is about 4-5 inches long in shell length and then finding a nice pond to release him (but ONLY if you live in the southern parts of US where they are native. becoming common in northern climes of US as folks have released pets and now are maintaining stable wild populations and outcompeting native northern turtles such as painteds).
for food, dont just feed greens-baby turtles need lots of protein...go buy some freezedried tubifex/bloodworm and see if it will eat that. my turtle loved it when she was a baby. they take more and more veggies as they age. but as i said need the protein as they grow. and only feed as much as will fit into her neck (estimate volume) once a day. no need for a heater if it is above 65 (although others im sure will say differently-this is a wild turtle-not a farm raised pet and will be fine. that heat lamp likely is doing a fine job of keeping water warm anyway being such a small volume.
if it seems to have trouble staying sunken dont worry. takes a few months for them to lose their buoyancy and be able to stay at the bottom. keep water level about 2-3X deeper than the shell is wide. avoid planting it as you will quickly become frustrated as turtles like to dig and will uproot all your plants. they do LOVE anacharis(aka elodea) and will happily eat it. just float it around in the tank. it wil lalso help soak up some of the ammonia/nitrates in the water) and then you can keep your lettuce for salads
over all things seem good. just giving a bit of info for you. but definitely get a uva/uvb bulb for it! and a big filter(around 3X larger than the water volume you have).
good luck with it! turtles are a lot of work but are really great pets!!!
cheers