As a relative newbie and very occasional user in the world of fretless, I would recommend starting with a decent but cheap factory fretless - something like a secondhand Squier VM Jazz or (my preference) Westone Thunder 1A fretless - to see how you get on. You may not get on with a fretless or, indeed, you might take to it. In either case you can move on the bass for very little, if any, loss of cash to either upgrade or forget the idea. But defretting a bass, even if done professionally, will almost certainly devalue it. IMO etc.