You have a sort of Grail bike---the Holy Grail of knock-around, ride-anywhere commuter/fun bikes. With that Walmart bike for $80, after you take it to a shop to get the wheels tightened and trued and the cables adjusted, you will be at $130. And the bike, if you ride a lot, will last a couple seasons …. Maybe.
Put $130 into the Hard Rock and you will get another 30years out of it.
You need a new cassette? Or do you need to clean the old one?
New chain? Yeah, but I’d bet the old one could be cleaned and used for a while---enough for you to find out how much fun riding can be. Eventually you will need to replace both …. But that’s like tires, tubes, and cables … stuff wears out.
If you plan to ride, investing in some simple tools makes a lot of sense. or about $20 you can get a chain whip and a cassette lock-ring wrench, which will fit every bike you will ever own. You definitely want a multi-tool with a set of allen wrenches so you can do little repairs on the road if you need to—another $10 or $15. A couple tire irons and a mini-pump---but you can wait on those until you decide if you are going to commute or ride regularly.
For a start … I use gasoline or mineral oil on a Really rusted part … Simple Green if it is just dirty … and a wire brush to knock off the worst of it. Though it isn’t as easy, you can do a lot of cleaning on the cassette and chain without even taking them off the bike.
You can get a rack and a bungee cord for not a lot of money at Walmart or online, or you can get a bag which hangs under your saddle …. Really cheap if you don’t mind waiting on shipping from China, and still cheap if you shop around on EBay.
I won’t lie. If you plan to ride a bike, you are going to Need some stuff. Riding a bike isn’t free.
You will need the bag or a rack and bungee, the mini-tool, tire irons, a mini-pump, spare tubes, and some oil----don’t spend millions on “exotic” lubricants, gert tri-flo or something. You might want to get a patch kit to repair tubes. If you have a lock, okay … if not you will need one. And as people have noted, you will need to replace tires, cables, brake pads, chains ….
Cycling is still the cheapest and IMO most enjoyable practical way of getting around …. And as with most things, the bottom-dollar, loss-leader, cheapskate special is not the best deal.
I tried buying sneakers at the Family Dollar once. Wore them about three times, then spent five dollars more and got some at K-Mart. The difference between shoes and shoe-shaped objects was immediately apparenttoo cheap was no good. The shoes hurt to stand in, and hurt worse to walk in. (I wear the same exact model of K-Mart sneaker now, 30 years later, that I bought then. Flashy names and labels mean nothing---performance is everything.)
I used to ride junk bikes. I got away with it because I always had three or four in the garage to tear down for parts, because they were always breaking down. I had tools and did my own work, but it seemed for every three or four commutes I needed to spend an evening fixing or replacing something.
Eventually I saved up money for a real bike---a Bridgestone MB4, similar to the Hard Rock---and suddenly I stopped being a bicycle repairman and was just a bicycle commuter. Nothing broke, everything worked. How much I saved in hors of labor and scrounging for parts …. Plus the joy of knowing the bike would always work---I never had to wonder what would break next, because Nothing would.
So … if you want the cheapest possible bike for the short term … sure, the Walmart bike will get you down the road. Be Very careful, though … they are often assembled by people who have no idea what a bike is or how it is supposed to work, and generally the cables need adjusting (shifting and brakes) and the wheels need tightening and truing.
There is no guarantee that any part of the bike has been greased, or that all the various bolts have been tightened. It is worth checking.
And likely----no guarantee, but most people who have experience riding Walmart bikes have similar experiences---lots of little stuff will go wrong, fall out of adjustment, need attention …
If you just want to cruise around the neighborhood on smooth pavement for a little while each evening, the Walmart bike might serve you well. No guarantee.
On the other hand, you could refit the hard Rock for about the same money or just a little more, and have a bike which will last for as long as you want it and which you could sell for $125 once you are done with it. You will Never be able to sell a Walmart bike if it is more than a couple days old.