They need to be pretty level, probably within a few mm. The tubing needs to be 1¼-1½" and the sizes about Length: 47cm Height: 30cm Base Width: 30cm.
The one in the picture is unfinished and up for £50, they go upto £100 for some and above for real top quality sets!
Your best get if you wanna go at it yourself would be treat/remove any rust spots, run over the repair and around with something like 180/240, etch prime then high build prime. That will fill most of the pitting from the rust. Once you've primed I'd rub primer down with 600 wet and dry till your happy with the shape and it's pretty seemless, then use a scotchbrite to go about 6-8 in a big circle around your repair. This will provide a key for your blend that you can then buff back up.
Light coats of colour to start with on your repair then go a tiny but further each time till you've achieved coverage and happy. Give it a day then run over with either a buffer or compound by hand and you should be good to go!
Id pick up a can of blending solvent/fade out too if you can, I doubt your colour will fe available in basecoat due to it's age, so it'll be either a MS2K aerosol or maybe even cellulose, notoriously hard to blend out of a rattle can but fade out shoud help. It's basically just a very slow drying thinner, apply a dust coat around your repair before you colour and it should soften your old paint up making blending abit easier.
You might find someone to do it at a reasonable rate, but there's a lot of people that'll quote you pretty high as it's not a straight forward job for a bodyshop now, most are happier to paint bumper corners and full sides ratter then small blow ins. Then there's others that'll want to do a shell off resto..