Welcome on board doco, can I suggest you ask this question in the racing section, you're more likely to get a useful answer there. Please though, hang around the cafe, have a tea, tell a lie about the guy next to you, and get your best punning head on.
I could be wrong but I dont think the changes are not huge unless it a rainy day rather than dry. Generally not much is changed unless the difference from say racing at paris roubaix to a normal road course.
I see your point about getting parts on the cheap but I dont reckon you can get a lot is sold and what they have is well used.
I did hear I think years ago that some of the lower budget teams use lower spec stuff to keep costs down (ie shimano 600 for racing as opposed to dura ace for promo stuff)
I think sometimes British cycling have some stuff for sale on the bc sitr
same as you should do really. By inspection. In a stage race the mechanics wash, adjust, inspect the bikes each and every evening. They inspect the tyres, looking for obvious wear, cuts and blemishes and replace those which fail inspection.
I remember a post on one forum where someone had asked what Michelin 'Service Course' meant on ProRace tyres.
It was explained that it meant they were the tyres the pro teams used.
'Cool' he said, 'so will I know which team it was who used my tyres, which rider ? Will they be nearly brand new or really worn ?'