Upon looking at the pictures, it looks like the graphics would be a pain to make. From my own experience sacrificing the 3D graphics for 2D saved literally weeks of time. Can you design your own textures, sprites, and all that other stuff? It's not hard to learn, but it'll at up a ton of time. You'll lose interest if you're not devoted to learning. You'll need to crate art for walking, running, dying, standing animations for every interact-able feature in your game. This will take a lot of time and patience, but it's easy to learn how to do. GMLite's tutorials let you skip this process, fortunately.
"Over-view" and "Side-scrollers" are pretty easy to make. I can easily see you making an overview-based game in a few days, except for sound and graphics stuff. Very easy to program. All you need to do is select a few spots for spawning, health/ammo/whatever packs, maybe some checkpoints, and anything else. If you want to get creative and even make a teleporter or something along those lines it would only take a few minutes to do the programming side.
Vehicles are a pain to create. I've only tried a couple of times and every time I got mad and gave up. They tend to get bugged easily and take forever because you have to adjust everything you made fr the vehicle to work. Vehicles = headaches.
Never, never, never, never, never make a game into an MMO unless it becomes extremely popular, like Colony. Too many extra steps are involved. I could go into detail, but since you don't want an MMO, I won't bore you.
NPC/player communication isn't too hard, but it might be a little advanced for someone of your expertise. It doesn't sound like it would be hard, but it's prone to bugs. Dialogue works like this:
>Player clicks NPC/walks up to NPC/shoots NPC/whatever
>Dialogue box created above NPC's head
>Text is created
>Dialogue box and text are eliminated either by player click or timer
>Dialogue box and text are created above player's head
>Repeat until conversation ends
To do each of these steps, you have to create the boxes, write the text, make timers, create cause-and-effects scripts, write "emergency" code to fix bugs, and do a few other minute things to perfect this. This process is achievable on GMLite, but in a program where you actually need to write programming it gets hard.
Same thing goes for leveling. To be honest, I don't know a lot about how levels work, but I think you have to use scripts similar to the scripts that store scripts on your computer. You also need to make a counter to keep rack of he player's progress, and make cause-and-effect scripts to make the player level up. After you do this, merely allowing the player add his/her xp to certain skills to make them higher is pretty easy. Making the player stronger would be unbelievably difficult for someone like you. I'm sure you don't know how to make your characters stronger after they level up. That takes about a thousand more steps. Omitting the level system entirely would easily cut production time in half.
Sound is probably the easiest step. Record the sound on your computer and insert the sound into the game. For example, if the player presses the spacebar to shoot, then pressing the spacebar would create the laserbam, make it move in the direction you're standing, and will make it move at a certain speed. It will also play your recorded sound.
