1) A ML contract will be for 3 seasons at $327K, and won't make a difference unless the top two bids are extrmely close. MiLB w/ invite costs you nothing and is better than regular MiLB. If you use a ML contract, you're going to burn all of his options (not a big problem for a star-quality player) and use a 40-man spot for his entire minor league career.
2) You can offer him as much bonus money as your prospect payroll will allow. If you check past years in a bunch of worlds you will find a few $30m+ bonuses. To see how high the bonus will go on this player, check the league's Budget page, and see who has how much prospect payroll . If no one has transfered any money into prospects, then the bonus can't go over $20m. Also, if it's later in the year, check to see who has spent what on other IFAs and the draft, to know what they have in prospects.
3) Don't go straight to your max bid. Sometimes you get lucky, and no one who sees the player has a ton of cash, so you can get him a bit cheaper. The exception to this is if I think my max bid will come up a bit short. Then I might go straight to it and hope to run off the competition.