These people
http://www.carpanels.co.uk/index_catalogues.htm
have 2 door arches, but if you can't get a 4 door arch you could cut one down around the door apature, that would give you more work making the bits up for inside the door.
I did a bit of shopping for the Manatee with them recently
If I remember rightly a wheel arch doesn't count as structural for the MOT, except where it closes the sill, or is near suspension mounting points, so if its rotted up to the shocker top it would count.
That generally means as long as the sill is closed and it doesn't have any sharp edges to injure pedestrians you can get away with most things.
Some of these

save a lot of time. Screw everything together first, and only weld when sure, that way if migging you can draw round edges and make sure you clean up well in advance or it will be horrible.
Get yourself a Knibbler, there are cheaper ones than the turner knibbler
http://www.machinemart.co.uk but at least you can get spares for these (order a spare DIE because if you are unlucky you can brake one and they are special order. Cheaper alternative
http://www.frost.co.uk/item_Detail.asp?productID=8015&frostProductName=Nibbler
A nibbler isn't necessary, lots of people use the grinder, but more precise AND saves showering your car in grinding sparks, you'll still need a grinder but it's easier on you and the car.
Just my opinion but DON'T use the whole repair panel if you can avoid it. Seams rust, so if your first cut is at the maximum size when the seam rusts you can't cut out bigger, or, if someone runs into your arch you can't go bigger which means undoing what you have done and that can be hell too.
One of these
http://www.frost.co.uk/item_Detail.asp?productID=8440&frostProductName=Joggler%20Hole%20Punch can be used to put a step in the edge of the panel , saves a lot of filler going into the side of your car. Mine is 20+ years old now, so not cheap but if you do one job every other year with it it easily pays.
Lots of cheap repairs people spot weld the repair panel on top (especially 2 door cars) and use a bucket of filler and don't even repair the arch just cover the rust... YUCK. On a 4 door that kind of bodge will stand out a mile away as the door skin won't be in line (unless the door catch is adjusted)
The inner arch can be as fancy or basic as you like from something from one formed piece either made yourself or from DR Manta (remember you don't have to use all a panel you buy if its more convenient not to) to separate sections welded in place, either just touching and spot welded to the inside of the lip of the new arch edge or fancier stuff folded with a return (but still small sections, less skill and time to make). Small sections are best made with the rear arch screwed to the car, you screw thenm to inner arch and each other. Then remove outer arch reduce overlaps if need be and it you already cleaned up get welding the inner. Spot it first (bits to each other not the car) until you have your repair panel. Try outer arch regularly. When happy spot it to the inner arch (doing this with outer arch off allowy you do do most welding without trying to weld up hill)
Eventually the inner is finished and noy you have to weld the outer. Start woth regular spot welds every 3 to 6 inches, then you keep going round working 3 to 6 inch apart so you don't do too much in one place and cause heat distortion of the rear quarter.
Basically an arch is one of the simpler repairs, but unlike a wing has the potential to make your care look an a*se and not be easily changed (another reason to not use the full panel? potential second shot at it?)
On the other hand if you can't afford to pay for a bodge (a proper job would cost a fair chunk) or just don't want to if its have a go or scrap it what do you have to lose?
I would do what the other bloke said though and get the carpets out / get under it / wings off look at A pillars / look at boot floor, dips under the rear rails rot like hell but sometimes look good due to sealer.
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