As long as you have an Instructor or someone very expeirenced with youngsters to be with you at all crucial stages you'll be okay.
It may just be me but by the time I was 17 I was working backing and breaking TB's at a racing yard. I was fearless with them and felt like I developed that special bond with many of them. When I was 20 I bought an unbroken but very well handled 3 year old Irish Draft x mare ... I had been backing horses for years but once I got her to the livery yard and was on my own I fell apart. I had a wonderful instructor to help me but at £30 an hour I couldn't have her with me all the time. The mare was very brave and generally very sweet but we still had problems along the way ... I suffered a huge confidence crisis and ended up paying a fortune to have her professionally backed and schooled ... I'm not saying that my experience is common or that it will happen to you, I'm not even sure why it happened to me!
I ended up selling the mare to a lovely home and still see her every now and then.
I bought a 6 year old after that - he had been broken but was green and needed schooling on ... I had several years of successful competing with him, he's still with me and is still ridden 5 times a week but we don't compete anymore.
Over a year ago now, I bought a 5 year old unbroken shire mare. Despite their reputation for being laid back and quiet, she is anything but ... every horse has their own personailty! She was a super star to back, she was turned away and bought back at the beginning of this year. It's taken 9 months to iron out her issues, she's tried on every trick in the book and I've felt like giving up more than once! I'm glad I persevered as the sense of achievement is undeniable!
If you decide against something unbroken, a recently backed youngster would still give you that sense of achievement.
A two stage vetting in my area 18 months ago was £130 and bloods were £48 extra.
HTH
Loosey