For the MK 5 you need a HP seat and O-rings. VDH is a good place. I buy my o-rings in bulk (10-20 of each) from The O-ring store. You have a multitude of choices in duro and in material. I get my HP seats from VDH.
For the 109 and 156 VDH is the only place to BUY newly made diaphragm covers and exhaust tees. He had them made from silicone. I got impatient before he offered them and made my own diaphragm covers and replacement tees. I will buy any needed further tees from VDH. For more routine overhaul you will need a couple of o-rings, a LP seat (if you have the duro poppet) or a new poppet if you don't. The other option is to make your old style poppet accept a replaceable LP seat. Check out the DIY section for instructions. My method has worked well for me. AWAP makes his own LP seats. Again these are available at VDH.
Tools needed: A curved spanner with IIRC at 1/4" pin for opening the ambient chamber on the MK5, a yoke nut, bullet tool, flat spanner for the HP seat piece, regular wrenches, screwdrivers, hex wrenches as appropriate, a vice (padded) or bolt to hold the 1st stage, lube (Christolube or like), and maybe a pair of pliers. Invest in Harlow's Regulator Maintenance at the very least.
I have converted my fleet of MK5s to DIN by adding SS washer(s) to make the SP universal DIN kit fit. Again, details in the DIY forum.
Good luck.