g'day there mate,
I'm in the same boat, sort-of. That is, I had twenty years off MX, and came back into it a year ago.
The first QVMX meet I went to for a look was enough to hook me.
I'm not a Suzuki expert but I don't think you'd have any problems finding parts for that model. There is virtually nothing that can't be found via the internet these days. I ended up finding the long sought after airfilter frame for my YZ125 in a Yorkshire bike wreckers.
DDR have a reputation for bikes at the expensive end of the spectrum. I don't know what they're like mechanically, I get the impression they give them a once over, check compression etc, but basically they're "as imported".
I personally would either buy a bike that someone is racing at present and you know works, or build one up yourself. It can be a false economy to buy a running bike and then have to start replacing all the parts that were at the end of their lifespan when you bought it. At least if you build it yourself you know the bike from top to bottom, and it's a bloody great feeling to start it up for the first time when it was once a pile of bits.