Rear back plates might be a pain to replace as you may have to remove the wheel hub to get to all the bolts properly ... and that is a right nightmare as the hub bearing is likely to split ... so you need a new one of those (or two!).
I don't think that they comprise any safety risk ... they just keep water and stones out.
I'm in the process of rebuilding my greenserpent website .. it's on http://greenserpent.unospace.net/
... as my old free one is cr@p ... This is still very much "work-in-progress" but if you go to http://greenserpent.unospace.net/download.html
and scroll down, there is a pdf version of my how-to on changing the brake pads and discs and bleeding the brakes and clutch. It's not as difficult as you may think! Torque settings are daNM ... so multiply by 10 for straight NM rates. (daNM is what the old DTE/eper quotes).
Yes you do really need a brake cylinder rewind tool for the rears, but it is possible if you are careful to do it with other things! Just be very careful near the piston seals, etc. know have one as I have to do a Pug 206 as well, but in the past I used some dodgy techniques! You will also need a 7mm hex socket or brake spanner for the front caliper bolts.
Good place to get reasonable priced branded parts is buypartsby.co.uk ... it's been a few years but did the front and rears for under £100! Check whether you have solid or vented discs as the pads are different and the rear brakes are small 251mm discs with pads with springs, or larger discs with a different type of pad. The calipers will also held: it they have "splines" they are smaller, if smooth larger discs.