I'm not as fearful of leaks as others are on this board. I don't think signs of prior leakage has to mean "disaster waiting to happen - run away". BUT, I do think you have inspect those signs carefully to determine what the potential impact has been/will be.
My used TT had signs of leakage around the windows. I saw the attempt at sealing to counter it, and it was very sloppy. The owner didn't remove old sealant and just gobbed the new stuff on top. Needless to say, the leak was NOT fixed. I spent a day removing old sealant around all the windows, added trim over the screws, and re-sealed. No more leaking. The prior damage was minimal and didn't affect the stability, etc. and it didn't turn the TT into a disaster.
With my clipper, there is staining around the A/C, but again, there was no structural damage, etc. I re-sealed all the overhead vents, etc. and there has been no problem.
So a stain doesn't mean disaster. Look at the "repair" work done. Was the old sealant removed first or was new sealant just gobbed on over and over? The wood base could have been replaced, so look close around the joints to see if there are signs of the leaking spreading down below.
As for the layout, I took a look and it's a nice layout. The only hesitation (which is minor) is if there is more than one person, it would be difficult for one to work at the sink and one at the stove - that's only an issue if you routinely have 2 people fixing the same meal. Other than that, the layout is nice. The dinette up front allows passengers to interact with the driver. The bed in back separates the sleeping areas with the kitchen if someone sleep in the cabover bunk or dinette. The dinette is close to the kitchen so you can use the table for more counter space if needed.