I couldn't do that but I should think you'd be paying more than £800 a month and I'd advise on results only i.e. traffic and sales, you can be the top of your best keyword or what you think is best but if it doesn't deliver then you're wasting your money.
Going down the affiliate route might be good for you, and I might be going mad, you may have seen two financial ads on telly recently... that wasn't a merchant, that was an affiliate - pretty sure it was anyway. And there's a few other affiliates that are bigger than quite a few merchants.
Your best bet would be to post on A4U and get some initial guidance from some of the pros, the networks may contact you as well, also the person behind A4U now runs his own advisory company and organises all the get togethers, of which I've not attended yet, they might be able to give you a pointer as well - nothing is free when you get started, I've also been told Jess who posts on A4U is very knowledgeable.
The good thing about affiliate marketing is the spend versus revenue/results but it's not always an happy story, take a few months figuring out how to get good affiliates, good proceadures and working on a good price to compete in your market, as with affiliates you're not only competing with your competitors and merchants but also the pay out, the EPC and how attractive you are as a partner for valuable ad inventory on affiliate's websites.
I'll keep a look out for you though in the future if you do decide on the AM route. I used to be a landlord, treacherous business for an 18 year old and buggered up my life a bit. haha
Cheers
Lee