Hi bouncibong,
I did my PPL on TMGs and then added an SEP rating; I also flew my skills test from Hinton so can probably offer some help on this.
I think it's an excellent way to do a PPL. The TMG syllabus is basically the same as the SEP syllabus, and in my opinion a TMG is a better basic trainer than your average Cessnas, etc., as it teaches you how to use the rudder better and generally improves your control co-ordination. Plus, the ability to turn the engine off and do a bit of soaring is a billy bonus and a useful skill to have.
I have flown the Super Dimona from Hinton (actually I believe the aeroplane itself is owned by the nearby Oxfordshire Sport Flying) and it's superb - looks brand new and has a constant speed prop, which is another good thing to have signed off in your logbook. The Grob 109 is a tailwheel aeroplane which is also a good skill to have and will further improve your handling.
More importantly: the instruction you'll receive from Clive at Hinton will be very good. I flew my NST and GST with him and have corresponded with him since and can testify that he's a very helpful bloke. The other posters are quite correct about not paying up front, and it's to Clive's credit that he's not asking for anything but payment by the hour.
However, regarding prices: don't let price be the sole dictator of where you train. More expensive doesn't always mean better, and - as other posters have alluded to - zero-to-hero PPL courses are always based on completing in the minimum hours (45); this is rarely achieved so definitely budget for a couple of £k over the quoted price.
As for converting onto SEP from TMG - I did this extremely quickly with a different flying club and there's not much to it, certainly nothing that should be beyond anyone at PPL level (only real differences are flaps instead of airbrakes and realising that a PA28 glides like a greased brick compared to a Dimona!).
Let us know how you get on!
Tom