You've got your location as Cardiff. Unless that's the nearest town and you actually live in the sticks, I would expect there to be hire companies nearby. If that's the case, is there really a market for rental from your location?
If you really see the potential, however, I'd reckon on it taking up a lot of your time. If you get a lot of customers, the administrative side alone will take up a lot of your time. Then there's the kit maintenance and testing every time it comes back in and goes back out. If you don't get as much business as you'd like you'll spend all your time trying to find a way to drum it up. You may find it never becomes a side business.
As for what to hire, that depends entirely on your budget and what your research tells you your potential customers will want to rent.
To narrow this down to something even close to workable, I'm going to make some assumptions. Most of those who work in commercial videos will have their own core equipment, so unless you plan on having the specialist things they may only occasionally need and thus won't own, I'm guessing your anticipated core market will be amateur filmmakers. If so, the Sony EX1/EX3 is brilliant in terms of what it will cost you to buy and what kind of performance it will offer customers. You'd need to be able to offer a wide range of accessories (for want of a better word) to stop people going somewhere else that can provide a full package. That means tripods and wireless lav and shotgun mics. Lighting (basic Redhead, Dedo and Kino sets) would be a bonus, but not essential if your budget is already stretched. The same could be said for things like dollies, jibs, basic Steadicam-type devices, sandbags, Magic Arms, Lastolites... Consumables like tapes (both DV and Gaffer) and gels (if you offer lighting) are where you can make a killing.
This list could get really long if I keep going before you narrow it down. It all depends on how much money you have to put down on equipment and what potential customers where you are based want.
Irrespective of that, customers will always want competitive rates and faultless service.
If you have no business experience, make sure you get some. Go on a course or get a load of books or read the all the business .gov websites front to back/top to bottom. And make sure you do plenty of research. Start by finding out if there is any competition near you. Find out who is actually going to hire from you. Find out if you're going to be able to access the necessary funds to lay down on the equipment you need to get you started. Find out how much your operating costs are going to be and how much any local competition charges and if you can afford to compete with that. (And bear in mind there are numerous internet rental
sites these days.)
There may be other people here who can advise you on the intricacies of the business side (then again, maybe there won't) but you're really going to have to furnish us with more specific information if you want more specific advice.
