Different engines have different TBOs. Some airframes are lifed, eg Bulldog, most are not. Engines vary in price, but generally un certified engines will be cheaper than certified ones.
A Permit is not a C of A. It is a non ICAO compliant permission from a national CAA allowing an aircraft to fly in national airspace. Having said that, you can fly in most EU countries and some non EU countries without specific authority, but you do need to check.
A Permit aircraft has more relaxed regulatory requirements meaning that you aircraft work can be signed off by an LAA inspector rather than an expensive certified maintenance facility.