Andy - Listen to the age advice of your elders like Bill - paying cash now is more important then ANY payments and interest you will pay on a boat you can't afford!!
I went from a 10ft inflatable raft with oars to a 10ft aluminium with a 5hp to a 14ft jonboat with the same 5hp, to a 17ft Tracker aluminium with a 40 merc and steering wheel which went 30 mph on my GPS and was FLYING compared to my 5hp! I ran that boat for 6 years after paying it off in 12 months, and if a great deal had not come up on my current 700lx I would still probably have that Tracker.
First define your budget that under NO circumstances will cause you to not pay your rent/mortgage, car payment, contributions to your 401k/retirement and food. Then decide what type of boat suits the water you fish (shallow flats boat, bass boat, walleye boat...) then look for a good deal in the price range you can afford and have it checked out by a competent mechanic before you buy it.
The 640lx is a great boat, Dan N. on this site has one with a 90, fit his budget for cash perfectly, fits in his garage, easy to tow behind his small SUV and on the lakes we fish in GA no reason unless its gale force winds he is not safe and confortable. My 700lx is only 8 inches longer. The 50hp like other says if propped right will plane, but you are probably looking at best in the low 30's for top speed. If you don't fish long lakes/rivers, or don't mind driving a longer tow to get to the area you like (I did the same thing with my 17ft Tracker), and its in good shape, go for it!
Trep