Iv'e just done a complete front and back strip down & rebuild. Here are a few of the problems that I encountered with the strip down.
(1) The flex hoses that connect to the caliper & the brake line where badly corroded and seized. On a couple of the calipers I had to cut the flex pipe and soak the joint in WD40 for a day to be able to get them out. The end that screws in to the brake line was a different matter, I had to cut off a section of brake line and fit new couplings. This meant I had to buy a brake pipe flare tool & make new lines. If you do have to do this make sure the flare tool makes the correct type end, it has to be a DIN ISO type flare.
(2) The bleed nipples where badly seized too and unfortunately the front ones snapped off. I tried to use and extractor, but they wouldn't budge so I had to order two new front calipers.
(3) The flex hoses at the rear are in two parts and seem to get a more corroded than the front. Because I was changing mine for HEL SS flex I wanted to try and reuse the connecting brackets so it took me ages to un-seize them.
I got all of my caliper parts from https://brakeparts.co.uk/ that included pistons, seal kits & new front calipers. The prices are fair & I spoke to the same guy a couples time who was very helpful.
I got my brass guide bushes, SS flex lines & Millers 300+ brake fluid from Hack Engineering. Again the guys there are really helpful. https://www.hackengineering.co.uk/product/millway-motorsport-brass-brake-caliper-slider-bushes-most-bmw/
I painted my calipers with Foliatec caliper paint, it comes in a good range of colors & is easy to use. one tin is more than enough to paint four calipers. I spent quite awhile removing the rust from the old calipers & I also had to remove the lacquer from the two new front ones. You don't have to prime them first, but I did apply an etching primer. I got mine from Demontweeks who is selling it for £20.50 instead of £29.99 https://www.demon-tweeks.com/uk/foliatec-brake-caliper-paint-and-engine-lacquer-243118/
The front discs are Brembo max & Ferodo DS2500
The rear discs are Mtec groove & Freodo DS2500
If you do go for brass bushes they can be quite a pain to get all the way in. If you use a G clamp like I did make sure they are seated straight because being made out of brass you can easily damage them. Make sure you apply plenty of geese then wipe the excess off when fitted.
When you rebuild your caliper make sure you apply some brake fluid to the piston & seal before you seat it back in the caliper. Also fit the rubber boot to the caliper before you try to seat the piston cos if you seat the piston then try to fit the cover boot you won't be able the fit the boot correctly.
The guys are right about Bleeding the system I have a Sealey VS820 Brake & Clutch Bleeding System. I flushed the complete system, even the clutch with 1 litre of new fluid. I thought that would remove all the air, but on a test drive I found the peddle to be spongy. Iv'e ordered another litre of fluid to flush again, i'll keep what comes out because the fluid should be new I just need to get the rest of the air out.
Hope this helps & I haven't put you off the idea. If you do need to replace section of brake line & don't want to do them yourself you can buy them ready made or get your local garage to do that parts for you for not a lot of money. GOOD LUCK
