I need to get my wheels done at some point in the next 2 months, that place looks good from what I saw on their website and prices are fair but a bit of a trip for me (3 hours). Grinspeed are good mate, haven't heard anything bad personally.
As for the car and the rust issues, well welcome to my world. I bought a car from up north, knowing pretty well that it will be a lot worse underneath compared to something from around where I live, however the price reflected that. Now as you know in my thread I have been trying to address the rust wherever I see it, it is the worst thing to deal with on an older car and I'd really rather spend the money and time on actual mods, but maintenance is key unless you want to scrap the car in a few years.
So it really comes down to two things. Money or time. Do you have the space and time to work on the underside yourself? If so it will not cost you much to treat the rust and underseal (something I will be doing next month too). While at it you can brush off all the suspension components and give them a lick of paint too (or remove to powdercoat if funds allow). If you don't have the time, tools or space then it comes down to how much money are you willing to spend. A reputable place will charge around £600 for the entire underside using high quality materials and several different coats. Here are some links:
http://www.jrclassics.co.uk/prices.php
http://www.rust.co.uk/rustproofing-price-list/p512929
For arch repair you'd have to get a different quote unless you want to go ahead with it yourself like I did (depending on how bad it is and if it requires welding).