If he wants you to invest your own money then I think you need to get a formal agreement of some kind, as an agent or distributor, with a term similar to your rental contract cancellation period. He could pay you a retainer, or agency fee. You could then be self employed, and you certainly don't need to register a company, especially if you will be the only worker (sole proprietor). As an individual, a landlord would be only too happy to deal with you, because you would be personally liable for the rent, rather than a limited company which would have no credit rating, and could go bump on him. NI and income tax is not so complicated, but you do need to provide for it when you are self employed. You really need to get some proper professional advice, either from a business advisor at a bank, or maybe the CAB. Your local CBI or Small Business Federation may be able to provide some help. But please don't dive in until you get the advice you need. The best of luck with your venture. Post again if you need more specific help.
By the way, I think you may find it difficult to persuade the tax people that you are self employed. in my opinion they will consider you as an employee - note that you say he will send you your "pay". I think it would be worth contacting HMRC. They are very helpful about stuff like this and they can advise you how to organise yourself.