As others have said, try a few if you can and see what feels comfortable - but of course you can't really tell until you can spin...
Single/double treadle can be one of the most important features. I can spin on both but wouldn't buy a double; they make you sit too straight-on for me and I get very uncomfortable if I spin for a long time. Others will only buy double treadle...
Upright ("castle") wheels are a safer bet than Saxony style (where the flyer and bobbin are offset, usually to the left of the wheel, sometimes to the right) because they are easy to use no matter which way round you use your hands. If you find you spin with your right hand forward, nearer the wheel, and your left hand holding the fibre, you may find it more comfortable to either have an inline castle wheel (where the flyer is directly above the wheel) or one that's adjustable (all the Majacraft wheels, and the Timbertops Lonsdale), or a Saxony where the flyer is on the right. (They do exist; I have a Traddy like this, spinningfishwife had her Timbertops Leicester converted, although some were made this way round, and Freyalyn has had her Haldane Lewis converted.)
You need to love the look of your wheel, so see if traditional-looking or modern styling suits you, whether you only like upright wheels or only Saxonies...
Material and colouring too; most wheels are made of wood, some light some dark; some wheels use modern materials.
What sort of spinning will you want to be doing? Fleece? Tops? Fine yarns or bulky? Art yarns or simple? Will you spin at home or will you want to take your wheel with you to spinning groups?
Looking at your location map, you've got Angus Spinners in Forfar nearest to you. It's not an affiliated Guild, I think, but that doesn't mean it's not good. In fact, I think [member=26680]Magnusmog[/member] is a member there; she sat with me in Spinners' Corner at the very first Scottish Smallholders Show, in Forfar, giving people a go. (And we've remained friends ever since
)
If you can't or don't want to go to a group, then my advice would be to buy a secondhand Traddy (Ashford Traditional) to learn on, because they are easy to come by, extremely easy to fix if there's anything amiss, a current wheel so parts and accessories are very easy to get; there are gazillions of people on the internet who can help you, and if you subsequently decide you want a different wheel, you'll sell the Traddy for what you paid for it, no bother. (Assuming you buy at the right price, of course - but we can help on that too.)
All the above also apply to the Ashford Traveller, which is a castle wheel. (The flyer is quite offset to the left of the midline of the wheel, though, much more so than on most castle wheels.) Secondhand Travellers tend to be a little bit more pricey than Traddies, but provided you pay the right price to begin with, you'd sell for the same if and when you decided to change.
(Despite all I've said about offset flyers, you won't discover a problem, if you have one, for a while anyway, so buying a secondhand Traddy or Traveller at the outset is still a good idea.)
Oh, and Traddies and Travellers have conversion kits for double treadle, so if you decide you want two treadles and don't want to part with your first wheel, you can upgrade
. (All but the very oldest models, that is.)
Traddies come in pre-1982 1-speed versions, which will usually set you back £80-£110; 1982 to 1988 2-speed, lovely wheels, usually fetch £125-£160; 1989-2010(ish) 3-speed, anything from £160 to £250, and the recent 4-speed wheels, currently retailing at £350-£430. If you buy an older wheel and want more ratios (it's like bicycle gears, you can ride anywhere on a single-speed bike, but it's easier to have a few gears
), you can buy a new 4-speed flyer for £35.
I know the Ashford wheels pretty well, but another popular choice which would similarly be easy to get help with and to resell if necessary would be the Kromski Sonata.
There are other makes and types that would be suitable but which are no longer made, so may be a little more tricky to get parts for. Two that are certainly good first wheels are the Haldane Lewis and the Westbury, both of which are saxonies (in fact they are very similar) and both of which usually sell for well under £100. I say good first wheels, but they are not limited in what they can do, and are not only useful to beginners. (I mentioned Freyalyn having one - she's a tremendously experienced spinner, but she loves her Lewis.
)
Shout up if I can help further
And let us know how you get on!