That sounds like fun!!!
As far as I am aware (from research and books), you prune apples during the dormant season (that'll be about Dec to Feb) but plums are pruned after they have fruited. I do mine in September. Apparently if you prune plums in winter you risk getting silver leaf disease - although I don't know what that is!
With really overgrown trees the main thing is not to try to do it all in one go. You can do the pruning over 2-4 years - the first year it would be best to remove the suckers and any diseased wood and any obvious crossing of branches (the aim is to open up the tree and let more light in to the fruiting branches).
The next year you can start to look at the shape of the tree and take it from there.
Of course everywhere you cut off a branch you'll get a load of shoots, most of which need to then be cut off the next winter.
Then there is the specific pruning of each fruiting bit of the tree and the formation of fruiting spurs etc - all rather specialised and I'm still trying to understand all of that myself! For that you need a good book with diagrams!