Fretless is not hard to learn. Even unlined, which I prefer. Just approach it without thinking you should be scared and you'll be fine in just a few days. Try and buy one with simple side dots on the G, A, B etc, notes; lined fretless with conventional fretted markings off the note positions witll confuse you. I would strongly recommend the Revelation unlined P bass, available from Bass Gallery in London. £199 new, so you can see if you like fretless very cheaply. I wouldn't be too fussed about the mwah sound. It becomes irrtating very quickly. It can be removed by raising the relief and action slightly, after which the bass plays just a normal bass.
Further up your budget you could look at the Ibanez Portamento (4 or 5 string), with unlined but with stub sidelines around £800, or lined there's loads of choce £350-750 in the Sire range