Anything more expensive than what you paid would be a risk since you were not even sure if you would enjoy bike riding, so your choice couldn't be more accurate. Congratulations on your new bike and welcome back. For your needs, the Downtube FS is a perfect bike just as it is out of the box.
Some of the more folder enthusiasts from this forum have transformed their Downtube into very performance-oriented, expensive machines. So later down the road, if you feel the need for improvement on performance, comfort, weight or whatever, the downtube will offer you a lot of flexibility for upgrades.
I personally believe that with or without the gell fusion your bottom muscles would end up conditioned to the new sport and you would get used to the saddle anyway, but one of the first things I would consider spending some money in the future is on a nice saddle.
Make sure you take some pictures of your new bike and share them with us (specially with those very pretty female NYC bike riders). I managed to have a Victoria's Secret Angel on my folding bike while in Rio de Janeiro, Brazil. Folding bikes ARE chick magnets!
Once again, congratulations on your new bike.
14R.