1. Should be discussed with both surgeons
2. There are two main transplanting methods, FUE and FUT. DHI from how I understand it is a method within FUE, so its just a way that they perform it. Looking at this is not nearly as important as choosing a reputable hair transplant doctor, with many years of experience, results and user experiences, like the ones posted here so you came to a good place. Many times things like DHI are a marketing tool, that make the clinic sound more advanced when in reality it doesn't mean much.
3. Your hair is likely getting greasy from sebum production, which could be induced from DHT. Before considering a hair transplant, you should have a better consideration for medication. Hair loss is progressive, and while your hair has held up very well into your 40s, it will only continue to miniaturize over time if unaddressed. The only two proven and FDA approved hair loss medications are finasteride and minoxidil, finasteride being the more important one as it reduces the hormone DHT which is mainly responsible for miniaturization. Finasteride and minoxidil will help to prevent further hair loss which will also aid in a positive transplant result. At the same time it is likely that finasteride will hellp reduce sebum production and your hair will feel less greasy.
4. My estimation would be somewhere between 1500-2500 max. That would also be best discussed with a reputable surgeon.
5. I don't have experiences with transplants in Turkey, mainly because I avoided the idea of going to Turkey when I was doing mine. There have been countless poor results posted on here and other forumns of botched turkish jobs. Not to say that there aren't good clinics in turkey, but you have to be very selective, and this forumn is a better research tool for a good clinic. It is very important you do not try to cut corners with your research or finances when choosing a clinic, relatively there are very few clinics worth going to for really good results. Don't make the mistake of saving money as your priority when choosing, this is a very delicate numbers game, and you want to use your grafts optimally, spare your donor and avoid getting a result you're left disappointed with.
6. The quality of the hair itself remains the same, it doesn't become more brittle or anything along those lines. You can treat like how you would normally.
7. As mentioned above, you should be on finasteride and minoxidil if you want to slow the progression of any further hair loss. Make sure to research more into these two. Both are very capable of increasing the density and thickness in your hair because they could work to reverse miniaturization. However, your hair won't get thicker than what it was organically, because hair thickness is determined by our genetics. So you can potentially restore some of your hair to its original state, but you cannot exceed that density. Note that these hair loss medications mentioned are better at halting further hair loss than they are at regrowing.