Welcome aboard to the best forum for these type of builds. A lot of standard construction equipment,excavators,dozer,dumps etc are available In kit form from several manufacturers, wedico, vario, rc4wd to name just a few. The others you've seen like loggers,graders, and quite a few dozers and so on are all custom built models.
Many builders start by using blueprints/plans from 1:1 units and scale them down to the correct scale. There's others that are just drawn up by the builder. Google helps in finding prints or you can start at
www.the-blueprints.com they have a lot of stuff there.
As far as starting on a budget you could always start with a conversion of a bruder toy there are a lot of those on here it will def be cheaper than buying a full kit. If you were not aware of pricing just to give you an idea wedicos,varios, etc can range from about $3000-$8,000 US dollars for kits depending on what options you want etc.
I'd do a lot of searching here on the site and you'll find tons of info on what you need to start and how to's etc etc .
--Dan