I don't have a star pick, but I can easily answer a few of your questions:

• The enchantments is has are efficiency 7, unbreaking 3 and silk touch 1.
• It depends on the user. I think it's usually worth around 800k-1million+
• Again, depends on the user. Usually it would probably be sold for ECD, but you can always sell your netherstars to get the ECD.
• Again, depends on the user. You're more likely to be able to haggle for a payment plan if you have a substantial down payment, offer to overpay, and have a trusted status.
• That I'm not quite sure on.
• With most (if not all) payment plans, you'll make one down payment and then either make one more payment on a later date completely paying it off, or you can make payments every week, every day, or something like that.
• That would be something you would have to ask whoever it is that is selling it. I don't think I've seen payments for star tools go for less than 100k per week, but I may be wrong. I would say 100k-150k per week on a payment plan, but that's just my guess. Every payment plan is different.

 

• What enchantments, if any, does it have, apart from eff 7? Eff 7, Unb 3, Silk touch 1...
• What will it cost me to buy? 750k-1mil
• Can I pay a user nstars plus money or one or the other: If the owner accepts it.
• Would people accept payment plans as I am poor at the moment() Yes
• What sort of money could I expect to gather when using it for possibly an hour and a half- two hours? 20k-25k
• If I did a payment plan, would I have to pay the whole amount before recieving the tool or would I get it and then pay week by week etc.? You can pay some upfront and then pay week by week.
• What sort of payment per week/month would a seller accept? 100k-150k