For the price of a GMC Denali, you should be able to find a decent used bike that will last you much longer, be more reliable (if it's been taken care of), and many USA or Japan-made 80's and early 90's frames are nice enough to be worth upgrading later on. A local bike co-op could help you keep your bike(s) road-worthy on the cheap and help you learn about your bike, if you give a more specific location I'm sure people could chime in if there are any in your area.

It's always good to have two bikes. Just in case. Or eight. Just in case.

For flexible and affordable cargo-carrying, get your rack on with p-clamps and think about adding either Wald folding baskets or a basket on top of the rack. One of those with some bungee nets and straps should make it easier to pack your backpack, any other bag you feel like commuting with, some groceries, and/or a 12 pack. Get some heavy duty zip ties and your basket(s)/rack can be modular and easy to switch around. I have panniers, saddlebags, folding baskets, top baskets, bar bags, and can use whatever makes sense for a given commute or day around town - but detachable baskets are the most flexible. I've gotten Wald baskets (folding and non-folding) for probably $15-20 each. Or get a milk crate and zip tie it on. Not the best for balance, but thousands of people have been doing it for years, and it's free.

Fenders if there's rain. Detachable are ok if you don't have the proper braze-ons (sounds like you do), but full fenders are best. They make life more pleasant, and will help keep you, your cargo, and your bottom bracket clean. Velo Orange still has 700C fluted aluminum fenders on special for $28, which is a hot deal - otherwise most common plastic fenders (Planet Bike, SKS, etc.) you can probably pick up for $30-35.

If you don't want things to get wet, wrap them tightly in plastic bags. You might look like a hobo, but there's nothing wrong with that. For your person, if you don't want to spend $$ on rain gear (some of it is pretty cheap, but the cheap stuff will probably get you just as wet from sweat)(though I do have an O2 rain jacket that cost $20 and works ok for me), just bring a change of clothes and a towel. Problem solved.

A small repair kit that includes a multi-tool and patches along with a small pump are a good investment, along with learning how to patch a tube. Extra tubes, tire levers. Of course, if you can ride along a bus line, that can simplify your repair preparations.

Depending on where you work or leave your bike during the day, you'll probably want a lock that gives you the confidence that your ride will still be there when the whistle blows. Rule of thumb: get a stout Kryptonite, On Guard, or Abus U-lock. Cables are worthless unless you're in an area with little to no bike theft, the thick chains are overkill unless you're in a big city or bike-theft prone town, but spending $45 on a good quality lock is a good investment.

You've already got wheels and a backpack, which is really all you need. Everything else is about adding some comfort, allowing you to carry more stuff, or keeping you a little cleaner and/or drier. You can buy stuff as you go, and as you figure out what works for you.