Having used spray.bike, I wouldn’t jump into using it for a nice bike. I’ll admit that I wasn’t fastidious about prepping the frame I tried it on (a balance bike), in fact I was kind of deliberately testing its claims about being unfussy to apply. I certainly didn’t blast the frame, just keyed it at best. But even with lacquer it chips very easily, both on the balance bike and on a brushed steel shower rail I used as a test specimen. I know there are people on here who have put a lot more effort into spray.bike jobs, though, and I would check how their frames are faring before committing a decent bike to it.
Like some others suggest, if you want a single colour then personally (YMMV) I would pay a bit extra and get a plain powdercoat: you should get a good finish and it’ll be durable. The last one I had done was £60 including sandblasting, I think; if you do a little research for coaters with good experience of doing bike frames, rather than the ones who cock them up, you should be fine. You’re looking at £20 for a lacquered single colour of spray.bike plus a lot of your time if you want a chance of a decent result, and if you’re paying to have it blasted as well then there can’t be a huge jump to a powdercoat if you pick your coater with care.
I’d be interested to know how you get on with spray.bike, but I’d definitely ask around before setting your expectations.