odds are your 'springs' ground water is fine; I'd test just so you know the parameters (of the ground water supply for your pond), esp if you're going to continue to use this as your supply. If you're going to continuously be putting in water via the sump pump, make sure you have an overflow, one that won't allow any fish/inhabitants to overflow with it. I'd put in lots of plants to help filter. If you're worried re constant filling (i.e. it affects your parameters in case the water is found to be very alkaline/soft/whatever) disturbing whatever pond parameters you want, you can have a tee/shutoff/diverter such that when the pond is full, you can divert the water elsewhere (you'll have to determine this elsewhere and fix it so it won't affect the pond+surroundings. I'm thinking drainage hose to somewhere far, far away or a french drain type fixture). This way you can use the spring water feed for filling if/when you want. But it might be that your spring water supply is just fine and you'll be having constant water changes (like a stream).
I don't have the numbers handy, but research what parameter range you need (i.e., ph, kh, gh, salinity) if you're you're going to have fish/wildlife in the pond. I know addy's well water is very soft and she had to add amendments to get started. In my case, my water (well, also) is approx ph 8.4 and hard but over time and many plants, is less and more mainstream re params. Hence why you should know what you're starting/dealing with and how to adjust.
Once that's established, if you need a liner (in case you don't want/trust any patching of leaks, etc) get one (45 mm EPDM). You'll need a pump and filter (bio as well as mechanical), if you're going to go the 'fish' route. And sizing will depend on your desired load. Typically, water should be moved 1.5 times per hour, but that's just a guideline, not a rule
There's more, but you can get started with these ideas and then do some reading here; lots of problems and solutions. I'm sure we've had some of yours.
Hope this helps.
Michael