I'd certainly look at giving these guys a go (The Accountants Circle). I've not had a website by them but if I want more than my current one I will be looking here first.
I'm always using the pdf info sheets they did for me, and they designed big banner for me which will go up at Gigg Lane (I'm an FC United Sponsor). Speak with Dave, he's always looked after me and I am dead sure you'll get value.
I'm not sure on testing anyone, but I guess it's a combination between what feels right for you and if you're confident with them. I'd imagine paying monthly is ok, but better not to be tied to a long contract if you're not happy.
I think this SEO stuff can be a bit overrated, and too many use it just as a way to get more out of you. With basic planning the website should do enough for you. Get the website then on your business cards, link to it when you post in forums - let people know about it as I wouldn't rely entirely on Google to do the job. If you're going to pay, you might as well use Google Adwords alongside your natural rankings. I think it's just the cowboys as you say which have really turned me off this stuff.
My brother put mine together pretty quickly, it's not big but it does the job I need it to do and I didn't pay through the nose for it.