Considering a tractor upgrade

Im considering buying a JD 455 with a very nice JD cab. It is presently set up with a quick attachment and 54"plow. It has no mower deck with it but is supposed to be very low hours on the Yanmar diesel less then 300. I am not sure of the year. When I originally contacted the owner he said sale was on hold because the tractor wouldn't start. He claimed the part would be about $150 and would only take about 5 minutes to put in. That was Wednesday. I ask about taking a look at it yesterday morning but it didn't work for him so hopefully tomorrow night will work better.
Now, I currently use a JD 420 that has been repowered with a Honda GX670, its a 24 HP engine and its performed great over the last 6 years since its been together. The hour meter doesn't work but I would guess it to have only about 200 hours since installation. I only use it during the winter for snowblowing. The chassis on this 420 I would guess to have over 1500 hours at least but has no problems. Currently it has a soft cab but the front and rear windows have cracks in the vinyl and the canvas is needing replaced.
My problem is I hate to buy something that will give me any problems since the 420 I have is rock solid dependable. But from what I have read the 455's are also rock solid. Add the diesel upgrade and an all but new cab is it worth the risk.
I figured I would take my blower and put it on the new 455 and the plow from the 455 to the 420 then sell the 420. It's not quite that simple but it's easier then building my Dart! I would estimate if all this would work out i would spend a difference of about $1200 - $1500 bucks have a newer tractor/cab.
So what do you think? Of course this deal hasn't been made yet so maybe it won't matter. I should know more tomorrow night I hope.
Rod