Well, I too am a senior citizen, but with a lot of computer expertise/background.
Two things. First is IF you do buy the $39.99 deal, you CAN/SHOULD burn a DVD with the ISO image. Keep that and the License number. There is NO requirement you use the license or install at any specific time. If nothing else you have an investment if you should decide to upgrade later.
Second is you can do a Dual Boot with the ISO, it just isn't straight forward. Google 'dual boot w7 w8', you'll see many posts on this, like these for instance :
Note that in all cases you'll need disk space to hold both. Installing W8 as a 'fresh' copy also will NOT pick up any existing applications you have. You'll need to re-install everything you want/need to W8.
First thing I'd do is even SEE if W8 is going to work for you. MS has a W8 Advisor link where you go to purchase, at a minimum, RUN IT and READ it. If you have a glaring problem of some h/w or program not working, then maybe W8 isn't for you at this time.
Now as far as W8 goes, I've installed it over W7 on 2 computers. First on my wife's 6 month old Dell. It was a very smooth install and we did have some hiccups along the way. The USB Network device did not connect all the time. Eventually removing the driver and installing again (it had NO W8 driver) did fix the problem. Other stuff was just because things moved and 'new' ways had to be learned to do what you wanted. However, considering it was my wife with limited computer expertise, I think it went well. Her only LOUD complaint were programs LATER discovered not to work with W8. Even the Advisor said they would. We had 2 major hits, now basically resolved. One was her GameHouse games. A majority did not run in W8. However, support worked with her and found many just needed a re-install, and some, well, no way, but they would work on older games based on popularity. The other problem was Norton Ghost. It failed to authenticate. Fixed by getting Symantec's System Recovery Desktop 2013.
Since she had good luck basically I decided to plunge in on my 3 1/2 year old computer. Advisor said all was OK other than a few I rarely use and some needing re-installs. Again, W8 over W7 as an update. Smooth process. Now besides the Ghost problem I had a Network Card on the motherboard problem. Did not connect on boot, but would if disabled and enabled. 1 out of 10 times it did connect on boot though? Now it got 'edgy'. I probably needed drivers. Well, mine and my wife's machines are Dell XPS's. OK, I went to support for my computer, Dell had NOTHING for w8! OK, since my wife's was really just a newer model of mine I looked at files for her machine. They DID have a network card driver. However, RealTek who makes the device had a newer one, installed it and it didn't FIX the problem! Installed the older Dell one, and it DID. Moral, you may need 'special' drivers from your computer vendor. Check first to see if they have them.
On my machine I have an ATI display card... but when I installed W8, ATI didn't have a W8 driver, so Windows install its own driver. I lost some control over color and control I have with the CCC from ATI. A few days ago ATI released the W8 driver. Control back.
I now 'sort' of like W8. On both computers we've NOT had a BSOD and it seems faster and more stable. It does have some annoying features, like SMARTSCREEN, but almost anything can be turned off. Start Screen, don't need it, but I like it. Live Tiles suck up Internet resources if you enable them, but with 60Mbps D/L speed I don't even KNOW they are active.
I'd say BACKUP your W7 install via an imaging program and then install W8 over it. Try it for a few days. Don't like it, go back to W7, all it costs was $40 and some time.
I'm not going back on either computer.