If it’s going to be your main sofa and only occasionally a guest bed, I’d be quite picky because a lot of sofa beds are decent at one job and annoying at the other. For your budget, I’d shortlist a few you can actually try in person rather than gamble on an online only buy.

From the replies here, Futon Company, some IKEA options and the Koala sofa bed seem like sensible places to start, but I’d also take seriously the comment warning against assuming every IKEA sofa bed will be comfy. The main thing I’d check is not just whether it feels nice for two minutes in a showroom, but whether the seat depth suits you, whether the mechanism feels solid, and whether it is still comfortable once fully opened. If it’s mostly for occasional guests, I honestly think it’s better to buy a genuinely comfortable sofa first and accept that guests may need a topper or air bed sometimes, rather than compromise your everyday seating too much.

I work in furniture retail at Depot Design in Brussels, and this is exactly the trade off we see people struggle with all the time.