I love the 500 but I dont think it is the ship for you for two reasons.
It is considered to be fairly poor for ag work because it has a very narrow swath due to the downwash when compared to a OH-58 206 or 206L. Unless you are willing to pay 25% more per acre, you need a bigger rotor.
500s are fairly hard to maintain correctly. A properly dialed in 500 is one of the most stable, reliable, safe and fun aircraft in the world but it takes a lot of time and money to get it dialed in. The guys who know how to do it right are few and far between and I'd say 9/10 of those guys work for the 500 operators, not repair stations. With the 500 not everything you need to know is the books, they need a lot of TLC over and beyond the manufacturers requirements. If you do decide to go for a 500 you will want to consider C20R engine at that altitude.
If I were you I'd be looking for an OH-58 or a 206L3 or a 206L1C30
Huey's are great, but I think I may be a lot more helicopter than you want.