I've read most of the replies and thought that this just isn't going to work if you have to buy his equipment at market value. Then as I thought about it I realized you don't need a $350k machinery investment to farm the 450 acres. He has $770 an acre in machinery cost. You could probably get by with less than a third, maybe even a fourth of that amount. If you didn't buy a combine maybe even less than that. 
You don't say if you work off the farm or not but, if you do have your living expenses covered you should be able to make this work. If you are sure you will have the 350 acres of rented ground plus his 100 acres you should have no problem getting a loan to put in the crop. A bank should loan this taking the crop as collateral. 
The equipment becomes a little complicated. You don't have the money for a down payment on your own so you might have to rent his for a year or two until you can save up a down payment for your own, financed at a dealership on older but workable equipment. Only rent the equipment you need! He can sell the rest.