Fantastic you've got the bug Nick, we could do with some more cars in Early Modern!
Definately wait until the new regs come out before you do anything drastic, interpretation of the rules/regs is key to making the best use of them and if your unsure of anything Stu the Technical Director will be more than happy to advise.
Regarding tyres a wise man once said pick your tyres to finish as if it were a 100% dry event, you don't want to get to day 5 with bald tyres and it being completely wet. This year we got 2 spares which could be put on for the 4th/5th day for 2wd and 5th leg for 4wd. but it could be different for future events.
My advice as a co-driver is find a navigator early and as a priority, spend time with them to understand it and driving together. If you leave it to the last minute and it will ruin your event.