73 300d

[imagesinthewind]

I have the chance to get a 115 for about $400.
It's got some issues, namely it idles fine but doesn't want to
go more than 20mph. It's said to have a knocking sound at around
3000rpms.
Seller says
"My hunch is that it's something in the fuel
delivery, I had her running a year ago and it sounded fine at idle - no
unusual knocks etc. It sounded really bad when you tried to go higher
than say 3000 rpm's, almost like valve float on a gas engine or maybe
rod's knocking, but I would think that if was rod bearings you would
here it at idle. It didn't smoke more than usual once it warmed up so I
think the compression is good."

Anyone care to give their opines as too what I may be getting myself into?

Thanks!