I don't have firsthand experience with Dr. Dorin so I can't help you there. While I don't think 26 is too young for a ht at all, understand that you're signing up for a lifelong job managing your hair loss and be ready to face that before pulling the trigger on surgery. Also understand that you're almost certainly going to need a second ht in your lifetime (more on that later).
Ask if they use microscopes to prepare and inspect the grafts. This is important even for FUE to ensure that quality grafts are being put into your head, and to ensure the right size grafts are being put into the right places (e.g. smaller grafts in the hairline). Don't be shy, ask to see the microscopes - any doctor worth having touch your head won't be offended. Also ask about how many technicians will be employed to place the grafts in your head. Fatigue is real - a single technician shouldn't be placing more than say 500-700 grafts into your head in one sitting.
Number of grafts: first off I want to say that I'm not a doctor and not an expert in this, so this is only food for thought and certainly not medical advice. But if I were you, I'd be thinking along these lines: assuming you aren't using topik in that crown photo and that's what your crown looks like naturally, I wouldn't touch your crown on the first surgery. You're maintaining it well, and by having surgery on it, you risk losing hair back there due to shock loss and by the doctor damaging native hairs. While it is true it won't be ideal to have a strong front with a weak crown, this is common enough among men where it won't look unnatural and won't give away the fact you had a ht. I'd come back to it when it's more thinned out. In the event you do in fact want it filled in, I honestly don't know how many grafts it would take. Going to the front: you're gonna want a conservative hairline to conserve grafts since you're losing your hair in your twenties and are thus very likely to need more work done in the future. I'm going to estimate that you need 1800 grafts to fill in bare spots and 1200 grafts to strengthen the center tuft, for a total of 3000 grafts. Maybe a little bit less, so let's say 2700-3100. But again, take what I'm saying with a huge grain of salt and listen to what the doctor tells you.
A final note: because you're already thinning, it's likely that even with medication, you will need a second ht for the crown within the next 5-20 years. A second ht is not just running back the first. If you go for FUE twice, the second time the doctor needs to be very careful to a) not over-harvest and leave you with a moth-eaten appearance in the back, and b) not take grafts out too close to the existing FUE scars, otherwise the scars might fuse and become more visible. Ask Dorin about his experience performing FUE on patients that had FUE previously. Ask him for photos showing the outcomes. Even though it's your first surgery, only go to a doctor who's skilled enough to successfully perform second round surgeries. Think long term so you don't have regrets later.
Cheers man, hope it works out.