Spares for Ferrets aren't exactly jumping out of the wood work but most things can be had if you ask the right people. Some of the basic consumables like spark plugs can be found quite easily and even the oil filter can changed for a cartridge type, if you prefer that over the nappy type which can be washed out in petrol and reused.
I suspect the reason for lack of answers is that most forum members have experience of the Ferret but not the BRDM.
For me I would buy a Ferret over a BDRM everyday as it is a vehicle that many people have experience of repairing, driving and using day to day. Its an armoured vehicle that you can fit in a garage, drive on your own and put a smile on most peoples faces when you drive by.
I'm not saying that you can not do the same in a BRDM you can. If you want to break new ground and learn things as you go I would say buy a BRDM but if you would prefer the experience and spares backing that goes with a Ferret that would be the direction to go in.
If I say that the BRDM would be a challenge I suspect I know which direction you will take
The V100 is an impressive vehicle if you can find one and for the right money