Used TI's are very rare and typically have a very good resale value, basically everyone wants a TI but some cannot afford a new one, so buying a used one is a nice alternative to some. When I bought my first TI back in spring 2010 I believe the price was around $4250, I also traded in my old Oasis at the time so the actual price was a little muddy, plus I bought a bunch of accessories at a very good discount ( ie... tramps, anchor, PFD's, scupper cart, lights, roof racks, kitchen sink, etc all at the same time and I don't recall for sure what the actual list price was anymore. Plus we had purchased many Hobies from that same dealer before and have a very good relationship (Economy Tackle in Sarasota, FL, "they are the best"). I like having my boats under warranty so I typically buy a new one every year or two and sell my old TI's (nothing wrong with the old ones). I think this helps others who can't quite afford a new boat but want one very badly.
Most of the improvements from 2010 to now are minor but some are significant, for example the newer boats have re-enforced scupper holes to help prevent scupper cart puncture of the tubes, the front AKA cross bar now has stronger welds, and the AKA bars are glued and riveted at the knuckle so they are now much stronger. The front hatch seal is now a much better design, the original design had round tubing super glued to the hatch liner (someone should have been fired for that original hatch seal design) I believe you can buy the newer type seals. I suspect from mid 2011 onward hulls were from a new mold, the new mold design makes the hull slightly thicker and stronger in some strategic areas. You can tell if your existing hull is from the new mold if there are black scupper hole re-enforcements in the front scupper holes, a front hatch with way better seals, and an embossed Hobie decal on the hull top near the front. Plus a few minor things like AKA braces and bars in newer models are anodized vs black paint.
There really was nothing wrong with the older original design, and that hull should give many years of trouble free service under normal use. If you only go out a few times a year, I'm not sure I would upgrade. I myself am a pretty heavy TI user and typically put on 20-30 miles per week on my TI's (I'm on my 3rd TI now), 50 weeks out of the year, pedaling around and scuba diving and snorkeling is my exercise program (yes that's well over 5000 sailing miles on my TI's).
In my position because I'm a heavy user I like to update my boats out every couple years so they stay in warranty.
But keep in mind if you were to have a catastrophic hull failure in the mirage drive area (very rare), or around the scupper holes, Hobies warranty policy is very soft on out of warranty repairs (especially with original owners), customer satisfaction is number one with Hobie and though your boat is now out of warranty, in the extremely unlikely event of a major hull failure Hobie will work with you on a replacement hull (unless you a complete idiot towards them (we all know some of those guys LOL)). Sure you might have to pay a few hundred dollars shipping, at the very least the price for the replacement hull will be nowhere near the real cost (that's just Hobies way of doing business). The reason for this is not because they are gods or anything magnanimous, but because the Rotomolding process is not an exact science, and out of every hundred hulls molded and finished out on their production lines, there might be a couple that have a slight visual blemish (usually internally, never anything structural). These factory seconds become the replacement hulls sold at a huge discount to loyal 'out of warranty' customers, and some replacement hulls, and possible some dealer demonstrator boats. What they do is a really really good and sound business practice that keeps everyone happy, (these guys are very smart and really know how to run a business properly).
Now they have their design and manufacturing all dialed in, I doubt very much there are more than a couple catastrophic hull failures per year any more (if any).
Really it's only the hull itself that can become a problem, (but as explained above Hobie is very generous about that) all the other components on the boat can easily be replaced with new replacements right out of the catalog ( ie... Mirage drive components, fins, AKA bars, pulleys, ropes, etc) all at very reasonable prices (they are definitely not trying to gouge you on parts like car companies do).
Bottom line, if I were in your position I would not upgrade, just use your existing TI and have fun with what you have, there are not enough difference between what you have and the newest model and the $2000 dollar or so delta to make it worth your while.
If you have one of the original 2010 models however there is one thing I would do if it were mine. I would take the AKA bars and fill the joint between the diecast knuckle and the metal tube with thin west epoxy. Basically set the bar upright, and pour the epoxy into the large gap carefully. This will prevent the rivets from coming loose after about 6 months of use (that's why all the newer ones are glued and riveted). Since your boat is almost new I see no reason to do a complete overhaul of the rivets and glue as posted by some users on this forum a while back ( I think Kayakbob), just pouring the epoxy in should fix the design problem.
To re-enforce your scupper holes if you want them stronger like the newer boats, is really simple, just glue in (with silicone) two 6 inch pieces of 1 inch PVC tubing into the scupper holes, this permanently fixes the problem only takes 5 minutes and only costs about $50 cents in PVC tubing. Or do like I do and just don't use the scupper holes anymore, a loaded TI is really too heavy for standard scupper carts. If you still plan to use a scupper cart I would definitely buy Hobies scupper cart cradle (I think they are around $30 dollars, and are in the catalog (near the scupper carts).
No need to do anything else, Enjoy