If you already hold a EASA PPL with SEP then you don't need to do a nav section - it's just a Class Rating test. So yes, the Venture will still save you a lot of pennies compared to the Diamona.
How long to convert? Depends on your experience of course. If you've got some tailwheel time it will help, and if you're pretty good at glide circuits and PFLs you're nearly there!! You could be ready for test in an hour or three...
To say any more might constitute advertising, so let's just leave it at PM me if you're interested...!