The idea is to get one, fix it up, and keep it in fuel. I set my budget for 1st year of purchase, title, tags, insurance, and fixing at $5,000. I actually came in under that to about $4,000 give or take a few hundred. I think I got off cheap because I had a lot of free help and was even given some pay it forwards on suspension parts that probably saved me hundreds.
You don't want your first experience to be a bad one. If you are set on buying one, inspect it good and set your budget. Be prepared to work on the truck and hire out what you know you can't do. I don't have the stuff to weld and it's been too long since I did welding to be any good at it. I hate painting and don't think I have the skills to do that. I can read a -20/-34 military technical manual, can turn wrench, and if I think something is too deep for me, I know folks who can answer questions and/or help me directly. So I hired out the welding and the body work which together cost well over 2X what I paid for the truck. My truck had some rot so I expected to pay for that. It's all a balance, you either pay a higher price for a nicer truck and HOPEFULLY pay less in intial fix ups. Pay a lower price for a junker and you have to pay for the parts and labor to fix it which can run a lot more than the cost of the truck plus the fact you aren't driving it yet. Mine took 6 months of putzing around to fix up. Two months were waiting for the body work to be done. I'm now at 18 months post purchase and just shy of a year since it hit the road and I am still at about $5,500 in total outlay including purchase price. I have yet to complete the interior and put the radio in it and a bunch of other stuff that I'll probably sink another $2,500 to complete. Of this cost, rust proofing and actual radios will probably be the biggies. Another decision for buying the truck I got was that I can work on it and successfuly fix things. That was a big decision for me to finally get what I wanted.
Also don't forget the cost of insurance and fuel. Your young age works against you for insurance but then again the vehicle isn't going to be a porsche so that will help save some. Diesel is going over $5/gal so that will be a big hit to the wallet. Mileage on a M1009 should be around 19 mpg +/-3 gal so figure out if you can keep this in fuel after you buy it and fix it up. Don't forget you will have to get it home after you buy it. The likelihood of driving it home is...well very unlikely. There's a lot of threads on this website about this very subject so you may want to do some seaching before you buy.
So key points:
Inspect it
Decide what you can and can't do to fix it
Have a plan to get it home
Determine if you can afford it
Make sure you can pay for the upkeep
Hope this helps.
btw I wanted a military vehicle since I was very young. I've always been enthralled by green iron. It wasn't until I turned 40 that I had my mid-life crisis and finally bought my "army truck".