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I wanted rust free but it looks like I'm going to change plan and get one that need the arches done. & thanks Very few folks replace the whole rear wing because it's so expensive. Usually once the arches are gone, you need to replace not only the visible bits, but also the leading edge which is hidden under the sideskirt, and also the rear section underneath the bumper. IMO the only way to repair rust is to cut it out and weld in new metal, which gives you a few options; patch repair - a decent panelbeater can make small repair sections, weld in, fill and make a decent job. Nissan cherry arch repair sections work, but need some fettling to get a decent fit. I used those and they're roughly £12 a side, plus fitting, etc. The rear section underneath the bumper was cut'n'shut using sheet steel. Tony (Jinxy) on here sells what is commonly agreed as the best repair panels and they cover all 3 areas. If you're getting a bodyshop to do the work this is probably the best option because it's least labour intensive.£100 or so, can't remember myself. Bodyshop is more likely £500+ depending on the skill/local labour rates, to do this properly isn't a 5 minute panel-bash.- these are the bits underneath the side skirts between both wheel arches. They usually go first at the rear arch & it's MoT fail because it's close to suspension/seat belt mounts. It's a complex area to repair properly because it's double-skinned inside the arch and because it's behind the side skirt it's often bodged. (mine was) The next place is to go is the front/under the doors and difficult to assess without the side skirts removed. I think the best panels are around £100, but you can use Escort sills with the usual fettling and bending to fit. £10-ish a side, plus fitting. Because they're hidden by side skirts, this isn't a bad option and fitting is much easier than the nissan cherry arch panels. Don't accept a cheap quote from a local bodyshop or use anyone who doesn't know Mk1s. It looks like an easy fix, but to do it properly is time consuming. (read expensive ! ) 
I'm still not sure which route to go down. pay less and get the work done. Wait for a good example to come up with the work done and pay the extra.£2.5k and I didn't want to go over it but a few weeks ago I saw a red '90 t-bar, 1 owner from new fsh & only 39k on the clock. It was 100% rust free but was £3k, so i left it a few days before calling as i didn't want to go over budget. I'm gutted as when i finally broke and called I was told it was sold the day before. 
Personally, if I had your budget then I'd wait for the right one to come along. I didn't wait but got a presentable car with the intention of just sorting out one rear arch. But the trouble is that once you start you end up going round the car. Ok, now it's presentable and even the MoT tester has noticed it's improvement over the last 2yrs, but it will never be as good as a mint example. £1500 just based on the hours I've put in, maybe even more. So, having been down the restoration route it just doesn't make financial sense - you'd be better off buying the best you can afford IMO..the SE region of the mk1 club is fairly active so you should be able to find someone close who could point at the bits for you. Any volunteers ?