Dish size depends on what Sat's you wish to find, a standard SLY dish would be fine if it's only 28.2e (Freesat) you want to find, but if your after a motorized system then you would be best off with a 1m dish so you are not as restricted with which satellite you want to find.
As for pricing this forum has a sponsor have a look on there for deals.
If you going to be buying new for the first time, then I'd stick with a UK supplier. In the event anything goes wrong you'll get a better response in both time and postage costs. Also if you buy your dish and box from the same place, you may even get a package deal or a discount, and it will include everything you need from cable to fixings.
I've only used one supplier to date so cant coment on others, but wizardsatellite.co.uk were good for me. My only gripe with their online ordering was that you sometimes have to enter a discount code for special offers, which I didn't notice at the time and missed out a little.
As for dish size, wether its fixed or motorised, will depend on your specific needs, your view to the satellite arc. A bigger dish will usually give you better reception in bad weather, but also be affected more by high winds.
Have a look on the sat listing sites for the channels you want to recieve, as there is little point having a motorised dish to pick up hispasat on 30w unless you speak spannish/portuguese and have a subscription.
uses google maps and will show you the direction your dish will be pointing for specific sats along with other information including very easy to read footprint maps.
lyngsat.com and kingofsat.net
give easy to read sat listings
As you seem to be up north, you may need a 1m dish if you can get away with it (they do look quite big) or maybe look at an 85cm fracarro penta dish as these seem to get good reviews, but i do not have any experience with them.
Now for the spanner in the works,
If you are new to satellite and are going to set up a motorised system yourself I dont think a TM-800 linux box may be the best "first box" pehaps a cheap Tm-5400 or similar of fleabay just to set the dish and arc up.
If going for a fixed dish ignore the above.
What channels/satellites do you want to try and get?