Hi there,
In short, yes, a combo is a head and cab in one. They have their advantages and disadvantages over separates rigs. Personally, I use separates and always have done, as they allow for a lot more flexibility, and I can mix and match heads with different cabs to experiment with different speaker sizes and such. I've used combos as well, generally, they tend to be a bit heavier, but if it's just for jamming in a garage, that's not the end of the world. There isn't really anything that's overly heavy and too much to lift, unless you look at an old Trace Elliot 4x10 combo!
A preamp goes in between the bass and the amp, these are what tend to give the eq, and some have other features such as distortion and compression. These shape the amps sound.
Tube amps tend to have a nice warm sound to them, I don't like them myself as they require a lot more care. You can't move them if you want to get out of a gig in a hurry, as they're fragile usually, and even more fragile when still hot (and the tubes get VERY hot), and though you won't need to replace the amp, eventually, the tubes will need replacing, and it's not the cheapest of jobs. But, on the other hand, if you look after it, it'll look after you and they can last a while.
As for the sound thing, a good solid state amp (none tube) will give you pretty much any sound you want. I've been able to get my sound out of an ancient Acoustic head, and my pretty much brand new Aguilar class D head.
My advice for buying something, is don't get anything that's likely to lose value. If you find it's not you after a while, you can sell it on, and hopefully not lose too much value. Also look for good deals, there's lots on here which would fit your budget and suit you well. I'd always go for something that's more than you'd need though, as it leaves room for expansion. A cab that'll take more power than that amp will put out would work well, as you can upgrade just your amp head for something more powerful, and keep the cabs, rather than having to upgrade everything at once.
As you're in Salford, if you've got any way of getting to Rochdale, feel free to give me a shout and I can take you through a couple of amp and cab combinations. I'll even pop the kettle on!