The thing nobody tells you in the shop is what happens after about eight months, when the mechanism starts to tell on itself. You hear it before you feel it, a slight looseness in the fold, and that's the moment you find out whether the frame underneath was built to take the weight of a mattress and a mechanism and a person, or whether it was built to look fine for a season and then sag. Check that before anything else. Unfold it, sit on the open bed, and bounce a little near the middle. If it dips or creaks where the two halves meet, that's the spot that will give out first, usually around month ten, usually on the night you actually have a guest in it.
If you've never owned one, start with the simplest mechanism you can find, a pull out with a metal bar frame rather than anything with extra folding panels or gimmicks. Fewer moving parts means fewer things to wear out, and a basic bar mechanism is easy to get serviced or replaced if it ever seizes up, which the fancier ones often aren't. Pull it open in the shop, not just halfway, all the way, and see if it locks with a definite click rather than a vague settle. That click is doing more work than any spec sheet.
These earn their keep in rooms that have to pretend to be two rooms at once, a home office that becomes a guest room twice a year, a studio apartment where the sofa is also the bed every single night, a cabin that only fills up in summer. For occasional guest use, a lighter frame is fine. For nightly use, you want the heavier commercial feeling mechanism even if it makes the sofa harder to move on your own.
Color wise, go duller than you think you want. Pale neutrals show the crease lines from the folded mattress within a year, dark solids show every bit of dust and pet hair, and the finishes that age best are the mid toned textured weaves, something with a bit of fleck in it that hides both. A tight weave also survives a mattress being folded into it thousands of times without pilling at the fold line, which smooth fabrics do almost immediately.
In a normal room it sits happiest next to a low side table that can be moved out of the way in ten seconds, because you will be moving it at eleven at night with a sleepy houseguest standing there. It also likes a rug underneath it, something that forgives the extra scuffing from the legs sliding during the fold.
What it does, at its best, is let one ordinary room quietly cover for two.