Re:
Ha. I was just thinking of starting a vintage cyclocross thread
Steel and aluminium cross bikes from the 1980s and earlier are quite hard to find, for a number of reasons. First, the sport was (to some extent still is) mainly a spectator sport, especially in its spiritual home, Belgium. This meant not so many production 'cross frames were made. Also, before the 1970's, a lot of riders just used road bikes with some adaptations like bigger tyres. You can find photos of guys like Roger de Vlaeminck riding 'cross races with ordinary side pull brakes for example. The 'cross specific bike is a relatively recent phenomenon, considering the sport itself is very old.
Second, 'cross bikes were ridden hard, even abusively and in unfavourable conditions (sand, mud, snow). I've come across many more damaged 'cross frames for sale second hand than road frames. Issues include cracked stays and bent head tubes (probably from impacts), internal rust, and other signs of hard use. Not to say don't buy one of these bikes, but be sure to see it in person and give it thorough check over (so no ebay purchases unless you really trust the seller).
Third, people have only recently caught on to the idea of 'cross bikes as a multi-purpose bike for different uses (on and off road, even touring). Most vintage 'cross bikes weren't built with this approach in mind.
Probably the most famous branded 'cross bikes were the aluminium lugged frames made by ALAN and Guerciotti. These were noted for having a very flexible ride, so offered a bit of comfort for cyclocross. The design was so popular that you could see a lot of riders in the 1980s using rebadged versions of these frames. Colnago made nice steel 'cross bikes but good luck finding one. For steel, some of the classic Belgian companies (Flandria, Diamant, Groene Leeuw, Plum) made decent bikes.
If you want to build up a solid bike for off and on road use, a good touring bike with cantilever brakes could be an alternative. Even a gas pipe frame (like the cheap Peugeots you mentioned) could be good solid option. The geometry and ride feel will be different. Cross bikes are built to be ridden fast in short races, whereas a tourer would have slacker angles, but it would still be a fun thing to ride off road if you build it up right. There should be clearance for quite big tyres, if you leave off the mudguards.
I just build up an old steel MTB frame with drop bars, and that is loads of fun to ride. It feels a bit more solid off road, but the drops are more easy to use for someone used to a road bike. I used a Breezer Sky steel frame as the basis for the build.
Surly make a nice all purpose cyclocross bike, the Cross Check, but a new one would be above your budget. Maybe you could get lucky and find a used one though.
As far as kit goes, 'cross bikes were mainly distinguished by having cantilever brakes, bar end shifters, profiled tyres, and sometimes a single chainring at the front. People also sometimes did weird stuff like routing the cables through the seat post and handlebar stem.
Good luck on this. Would be nice to make this into an active thread for people to share their experiences. Maybe even a sticky?
Johnny