Hi tinyd, like the sound of the afro-cuban outfit - well I've never played an EUB, much less a Stagg, but I made the transition from a slab to the upright about 20 years ago, never looked back. So my first reaction to your post was 'what the hell, just go straight to upright if that's what you want to play, shop around, you'll find one cheaper if you wait a bit', then I thought, 'well hang on, maybe you've got a point'.
Like I said, I've never played an EUB, so I can't comment on 'bad habits'. I guess an EUB will prepare you for the change in scale length and the angle of play, and would allow you to start gigging sooner. You can then gain experience and confidence, and work out whether it's for you. But, I doubt if it will prepare you for the physical challenge of getting a decent note out of an acoustic double bass. It's not just a question of technique, it really does need a lot more effort than a bass guitar. Plus, all the issues of amplifying a big acoustic instrument, that's knowledge and experience you could do with sooner rather than later. And (the real reason I started playing upright) - you can play it acoustically: when I first started playing I gigged regularly in a wholly acoustic old-time blues trio (guitar, washboard, upright bass), sometimes I still play acoustic at restaurant gigs.
I can totally see your reasons for considering the option as a step towards the real deal, and I have to confess that I'm looking at getting an EUB for travelling/flying (Eminence is getting my vote so far), but I'm glad I just jumped in at the deep end (pun intended). I'm looking forward to hearing what other players have to say, and to hearing what you decide and how you get on... cheers, Teej