As far as the photos go, I do not see anything on the Jensen 18 that would be a concern. The rear seat is not stock and there are some areas of erosion through the front deck cap but the gunwales and hull look fair and true. That boat is either Tuff-weave (fiberglass/polyester) or fiberglass depending on the date of construction which you could determine if you got the hull identification number (HIN). It show Wenonah’s center rib construction method (which is tough but no longer used). The boat will weigh around 66 lbs.
The Jensen is a very fast hull but not very deep. Not deep is good for not catching a lot of wind but it won’t be as dry in rough water. As long as it is used on protected inland streams and lakes and not overloaded, it should be fine.
If the Old Town is a Penobscot 16 it is a Royalex hull and will weigh around 65 lbs. The Penobscot 16 was a very nice design but a 16’ tandem could be a little tight if carrying two adults, a child, and camping gear. It might work but would depend on the size of the paddlers and the bulk and weight of the gear. I can’t tell what kind of repair was done. It looks as if something was gooped on to an external crack and painted over with spray paint. I would ask the owner if they know what was used. If I was considering buying that boat, I would closely examine the hull interior opposite that area for any sign of weakness or damage and push in on the hull at that point to see if it is soft.
When looking at Old Town Canoes be aware that those models which have three numbers after the model name, such as the Penobscot/Allagash/Discovery 169 or 174 were generally three-layer rotomolded polyethylene boats, rather than Royalex hulls. An exception is the Old Town Tripper 172 which is Royalex.
While the two different hull materials may look much the same, there are significant differences. Polyethylene hulls are much heavier than Royalex and harder to repair. Conventional epoxies and vinylester resins bond very well to the fabrics and resins that most composite boats are made off, but less well to plastics. This is especially true of polyethylene which has been notoriously difficult to repair durably in the past. I would pass on any three-layer poly boat that has been repaired. And be aware that any three-layer polyethylene canoe that is big enough for your needs is going to be very heavy. For example, a Penobscot 174 will weigh around 85 lbs.