Reddit reviews SunRace SLR 80 8 Speed Clamp-On Shifters 28.6mm Clamp Size

Upgrading it will be quite expensive. However, it'll provide you with a wealth of knowledge, frustration and, at the same time, fun.

Depending on your existing hub, you may or may not need a new hub to put your new cassette on.

With a wide enough range of gears on your rear cassette you can get away with a single chainring, so there will be no need for a new crankset/front derailleur/front shift lever.

As /u/fclbr said, you can choose downtube shifters in order to keep your existing brake levers. There are also bar-end shifters, and if you go for a single chainring, it won't be that expensive.

All-in-all, consider if this bike's right for you, size and geometry-wise, as your decision may be a bit rash. If it is, then I'd say go for it.

Rear derailleur - $20

Cassette - $20 - I recommend going for the 34t max sprocket. It'll allow you to stick with a single chainring

Bar-end shifters - $55 - they are 9-sp indexed, but they also work in friction mode, which allows you to freely adjust your gear

Downtube shifters - $14

Cables/outer cables are negligible costs.

You definitely need something there in order to get the shifters going. You can either use the drop down shifters, which I have found for decent prices on ebay, but also some at exorbitant prices. Like I have bought some 105 7-speed and dura ace 6-speed each for less than $10 but some people slap 'classic' or 'rare' on it and charge $50-100. You can also get some of these without buying the shift lever bosses, but it has to be the right diameter for the down tube, and I'm thinking the r300 tube is too narrow for that bike. But, measure so that you know for sure.

Also, what type of rear derailleur is on there? This will determine the type of shifters you get. The ones linked are 8-speed shifters, but I use with a 7-speed rear derailleur by locking out the last cog. It looks like you have an RSX on there that is 7-speed, but if you can find the model number, you can confirm this for sure.

Also, you can get these that screw on to the shift lever bosses so that you can run brifters like these. I did this on an old bike that had shift bosses for drop-downs, and it worked out nicely and was relatively simple, and the linked brifters came with cables. Or, you could always change the bar to a flat one and get something more economical from the mountain line, and the flat bar may be an easier ride if your dad is older.

Hope it helps!

Probably need more information on the bike. Assuming that the reference to downtube shifters means you have drop bars, which means thumb shifters are out.

If it's 7sp or 6sp, Shimano make stem shifters for indexed 7sp (I think 6sp are also still available, but you'll also need Shimano downtube cable stops for the cable housing from shifter to downtube which then guides the shift cables to the bottom bracket cable guide.

If it's an 8sp you could probably make these SunRace work with a shim on the stem.