690 minus 20%= 552 Fair sale price.
552 times 20%+ 110.4 There's your down payment.
Find a couple friends to take 30% ownership and 30% responsibility, retain 40%, and your share of the down payment is 44K.
(or 51/24.5/24.5)
Sounds much more doable, now doesn't it?
Of course you'd have to figure payments on the balance (relatively easy math) and operating costs (which should be available upon request from the current owners), and then figure how much you'd need to clear each month in order to break even/turn a profit.