Ok first I would like to address using your friend as an instructor (mentor). Unless he is an experienced instructor and in particular with the large gassers this depending on him to function as an instructor is a mistake. First problem is he an instructor at any organized field? You see many field will not let just anyone instruct and do not even remotely entertain the idea of just going off some place to fly with a giant scale trainer. Is you friend experienced with buddy box technique this is very important and a skill that takes practice and experience by the instructor and this has nothing to do with his ability to convey the needed knowledge and skills to you.
Next with the large gassor it would be a very expensive mistake indeed to attempt to not work with a buddy cord system and there are some fabulous cordless systems now that I use exclusively but Using the snag the box method is often fatal. Ya sure it can be done and I still do on occasion but not with an expensive giant scale airplane.
OK lets examine the economics just a bit and examine propellors, this just one of just about everything to do with 'giant scale'. I always tell my students to always carry several extra props and props for the typical forty sized trainer cost about three bucks. And the reason is prop strikes are common when you are learning That's the bottom line and when the props for your typical gasser are going to start out about 16 to twenty bucks. Props strikes that would not even stop you from flying with a glass filled prop typically used for training will stop you dead in the water with you GS prop. and the list goes on.
Unless you are prepared to work with an experienced instructor (and this is what several were hinting at above ) and with an appropriate buddy cord system then using the large gassor is a big expensive mistake.
That's the reason there is not much in the way of GS trainers and the two listed by Charlie and Quinn are just about the only trainers out there in that kind of airplane.
John