thanks LEE, I'm right in thinking f1.8 is a really wide aperture, so very narrow depth of field, yes?
also am I right in thinking that the f number is restricted by the lens, but the camera sets it, so the camera somehow 'knows' what f-number range the lens offers, and only lets you choose from within that range? so if you've got a lens with just a single f- number, you can't actually change it using the camera?
Lenses with a single f/number are rare (such as "Mirror" lenses).
The smaller the f/number means the more light passing through the lens. small f/numbers = bigger money.
Before paying big money though you need to ask yourself what type of photography you are interested in*
*There's no point in paying £4,000 for f/1.2 if you are going to put it on a tripod and take landscape photos at f/8.
All DSLRs I know of control the shutter speed and aperture (and ISO if you like) based on the other settings you select yourself.
In shutter-speed mode you set the shutter speed you want (a fast one for action) and the camera takes care of the aperture (and ISO if you like).
In Aperture priority you set the aperture (a small number for background blur) and the camera takes care of the shutter speed (and ISO if you like).
There will be a Program mode which selects a combination that works, but may not be what you want (sometimes it's not a bad idea to leave your camera in Program mode, just so you can grab a shot in a hurry). I tend to leave mine in shutter-speed mode at 1/250th second, because that is fast enough to freeze something if I need to grab a shot quickly. I let the camera worry about the aperture, I just want a sharp image without having to think.
Once you get to grips with the relationships of Shutter-speed / Aperture / ISO speed / Focal length of a lens then all the DSLRs allow you to set everything manually.
Normally, if I'm working in Manual, my camera is on a tripod and I have time to think.
Edit. A £200 Ebay Canon 1200D or 100D (better) will do all this (as will Nikons and others at the same price-point. It's just that I know Canon model numbers).