Buying any coach requires you to do your homework. You need to determine what you want and what you are prepared to pay for it. You also need to be knowledgeable enough about what is a good coach and what is a poor one.
If a little bit of dirt is enough to make you pass on an otherwise mechanically sound coach, then perhaps you need to rethink your expectations.
If you do your homework and are able to look past a little dirt and minor repairs to properly evaluate the systems and mechanical condition of a coach, you would realize that the dirt is a great negotiation tool that you could use to your financial advantage. So what if you have to spend a little time and a few bucks to clean a coach that looks exactly like yours will at the end of a trip?
If you only look at spotlessly clean rigs, you may be fooled in to buying a great looking rig that has an engine or transmission about to take a dump.