One last question what sort of budget would be practical?
Take enough Euros in cash to cover your fuel costs, even airports that take credit cards often (especially in Spain) "cannot get the machine to work". In our experience cash is often quicker and simpler than paying the fueller by credit card. Make sure you get receipts for fuel/landing/parking fees and keep these long after you get home as it's not unusual for airports to bill you for fees that you've already paid a year later.
We budget for each stop-over approx €30 landing, €10/night parking, €30 taxi to/from airport, €150/night hotel room (this can vary according to your taste of course). Add food and drink and entertainment (say €40/day each) and you should be OK. Food, drink and hotel bills can be from very cheap to very expensive - your choice.
Make sure you have a credit card with enough credit to get home by scheduled airline should you break down or to pay a local engineer to fix the problem.
As said previously if you plan well then the trip will be much easier thank you expect.
PS If you pre-book hotel rooms make sure you choose those with a decent cancellation policy (e.g. allow cancellations up to 24hrs in advance).
PS A very good tip I picked up from this forum is to always fuel up when you arrive if possible. Ask the tower when taxiing in to call the fueller. It also means that departure times can be a lot more predictable and less stressful.