This subject comes up often and there is always some pretty strong opinions on it…
First off personaly from the riding you say you do i wouldnt get a supermoto at all.
If you do decide you want one i would go for something like the KTM LC4 or the XT660. Not true motards and certainly dont have the grin factor a pure bred moto has but still has the basic characteristics such as ride position and ofcourse if you want to do a little green laning you can on these.
I wouldnt go for the DRZ (and yes i have rode one). Going from the VFR to a DRZ you will quickly get bored of the lack in power and speed. Yes they aint bad bikes but at best i would say they are a good first motard if you haven’t rode a big bike before. You can spend more money on vamping them up abit but not with your budget.
Now if you realy want the true motard grin factor get a pure bred moto. There are plenty of different KTM motards but the most popular seem to be the 450/525 EXC, there is also the Honda CRF450, Yammy WR450 and many more MX/enduro conversions. If its out and out torque, flickability and complete lunacy you want these are the toys to look for. They are great on the tarmac and in the dirt, just check out pics and vids from swaffham or any supermoto clips on youtube.
As for maintenance…
Your road ready supermotos… LC4, XT660, SMC 690 and so on do have longer service intervals. And im pretty sure you could service them easily enough yourself with a manual but ofcourse with a bike like this its good to have the service book stamped up which if you do the service yourself you aint gona get.
Pure bred motards such as the KTM 525EXC are peice of piss for anyone to service. Yeah they require more regular oil and filter changes (dont need to change the filters everytime you do the oil) but you aint gona be paying for the man hours to have the job done, unless ofcourse you realy want a service book stamped up for it but im yet to ever see an enduro/mx/supermoto with a service history. They arent as expensive to run as people seem to think.
Pure bred motards are easier to work on, less damage when dropped. Second hand spares are normaly pretty easy to get hold of meaning less cost in repairs.
Road ready motards are abit harder to work on, can do more damage when dropped (depending what one you get) and you could find it costing more to replace broken parts.
At the end of the day it all depends what you want out of the bike but from the little you have said i would say stick with the VFR or get an LC4