Take it from somebody who was looking for a vintage Triumph for quite some time and looked at alot of frogs before I found a princess: when they say "rebuilt" engine, 7 out of 10 times that means new pistons/rings and *maybe* a valve job. Rarely do people "do it right" and split the cases, clean the trap, replace all the lower end bearings, etc. For an asking price of under $3K, I would not expect that a comprehensive engine rebuild was done. As for the one for $1500, I wouldn't even bother looking at it, but thats my opinion. If anything is going to leave you stranded, it would be a $1500 vintage Triumph!
As for bobbing it out, if thats your thing go for it! I bought my #'s matching '68 with the intent of doing so but decided to go a different route. Alot of those Triumphs had the removable rear frame, so don't be concerned about "cutting up an original". As for getting used to the right side shift, it took me about 15 minutes to get used to it, and I'm a slow learner! If people can't switch between left and right side shift, kind of makes me wonder how they can function in life