Don't try to modify a street bike into a dirt bike, it never works out well unless you have a lot of time, money, fabrication skills, and time and money. Believe me, I invented turning street bikes into dirt bikes, jst not worth the trouble.
As far as good beginner bikes:
KLR
Minuses =
heavy, slow, not great on the street, not great in the dirt.
Pluses = Cheap to buy and own, hard to break, and can do a lot of things, just none of them well.
BMW F650, early fuel injected models (stay away from carbed ones, if there are any left)
Minuses =
Heavy in the dirt, slowish, BMW parts may sometimes cost more, you may only find ones with a lot of miles in your price range.
Pluses = Best single cylinder engine on the road, almost no vibration. Best street handling "dual sport" bike for the money, relatively trouble free, super internet support, dealers generally will get you the right parts the first try, if you need to take it for service, BMW techs usually know what they are doing. Can be ridden 2 up in relative comfort, has many hard bag options, easy to work on. ABS is nice to have.
DRZ 400
Minuses = It is a dirt bike, its heavy for a dirt bike, it way slower than a street legal 250 KTM dirt bike and weights probably 100 pounds more.
Pluses: Its a dirt bike, it is low performance/low maintenance, it can do most things a dirt bike can do. It's cheap. Also comes in supermoto version for having way too much fun in town.
V-Strom 650:
Minuses: It is pretty much a road bike, and by road, I mean road regardless of surface. Beginners will want to ride it carefully on gravel.
Pluses:
A reliable middle weight bike that can do more than most "street" bikes. Can tour with luggage and even two up in some amount of comfort, can pass cars while going up hill, cheap to own.
V-Strom 1000:
See V-Strom 650, but it's not nearly as good. Don't know why, they just aren't.
Ducati Multistrada 620:
Street bike, handles rough roads in urban or rural setting with ease. Fast for a little bike. I have personally drug my knee through a corner while passing sportbikes (on the track) numerous times on a Multistrada 620. My wife currently has one for touring.
Minuses = you must be somewhat committed to maintenance and TLC to be happy with an Italian motorcycle.
I can go on and on, if you want to know any specifics about various different bikes, I can probably answer them. I owned a Ducati dealership for 10 years, Master BMW, Ducati tech.
I currently own:
KTM 1190 ADV R
Ducati Sport Classic 1000
Moto Guzzi 750 Ambassador
Ducati 800 Monster (wifes)
Ducati 620 Multistrada (wifes)
Moto Guzzi 500 Monza (wifes)
Ducati 160 Monza (wifes)
1973 Honda CT70 (wifes)
and a pile of 1960's Ducati project bikes (1/2 wifes)
We're kind of into bikes at my house. Wife has 2 in the dinning room right now because we can't afford to buy any actual art.