How difficult, and how long are Doom (1993) and Doom II (1994)
Also can they be beaten without needing a guide or getting massively lost?
I've never played them prooperly before but I'm tempted to pick them up on PS4 since both are on sale
First of all, welcome to the club! :)
The PS4/XB1 port is an excellent starting point! It has all official WADS: Doom, Doom II, the two Final Doom campaigns (which are hit and miss in terms of quality to be honest), Sigil recently made by one of the old devs, but also various fan-made campaigns that Bethesda has been adding from time to time. It runs at a smooth 60fps, it has various tweaks to make the game run better on controllers (the game does support keyboard and mouse, however), it's pretty customizable all around. An excellent port, I've been playing it a lot despite regularly playing on PC already. There's even a pretty active speedrunning scene for it.
As for the difficulty, there's 5 levels. I'm Too Young To Die has less enemies, they do less damage, and overall few of the fights are actually dangerous, especially on the classic Doom and Doom II maps. It goes all the way up to the 5th one, Nightmare, which is basically meme level: enemies are faster, shoot faster, and they even respawn continously. The non-meme highest difficulty is Ultra Violence, a difficulty that is mostly feasible on classic maps for experienced players, although the Final Doom WADs can be tough as nails on that option.
If you have any experience with navigating in a first person game, you should be fine with the movements. Not being able to look up or down may feel weird at first, but it actually makes the game easier in a way, there's heavy autoaim to assist you for heights and you are less likely to get lost in the verticality of the levels. You can turn on autorun for moving faster. Keep in mind that the game was balanced in the early 90's when a lot of people didn't even use a mouse or a gamepad to look around, they would basically play the game in tank control mode, so to speak, stopping to turn manually with the keyboard. Nowadays it's a lot easier to get the hang of the movement, which is why the game can feel substantially easier.
Still, there's many ways you can progress. You can save at any point of the game, keeping multiple save slots for your adventures too, so if you find yourself dying a lot, nothing stops you form "savescumming": every successful step of a battle where you didn't lose much resources or health you can save, then reload if you did something in a non-optimal way. I don't think you will need this on the lower difficulties, but hey, the option's there. You can also mess around with the many cheat codes, with the no-clip cheat in particular that come very handy to learn the maps.
Speaking of the maps, navigating Doom maps can be somewhat tough if you're used to more linear shooters nowadays. Many of the maps are fairly straight-forward, but even more feature coloured keys to find, minor platforming sessions (especially in Doom II), switches to find that open stuff on the other side of the map, timed elevators and so on. If you want to find all secrets and items, you will do a lot of trial and error, pressing on a lot of random walls, trying to go through passages that seem closed but could contain something anyway. If you're only looking to finishing the levels, you will still need to keep an eye open and use the in-game map a fair amount to make sure you don't get lost. The game does a great job to visually distinguish most areas, but there's certainly some confusing parts where I don't exactly remember the path myself despite having played all classic maps many times.
But all in all, no need to worry too much. I have friends who are absolutely unfamiliar with shooters and are pretty terrible at them, but they still can manage to play Doom with the "tricks" I mentioned beforehand. If you still get lost, there's a myriad of guides, map breakdowns and videos to find your way - don't be ashamed to look up where to find that last elusive secret. You'll find that the game aged incredibly well despite the technical limitations, still providing a satisfying, adrenaline-fueled yet tactical shooter, where you'll blast enemies with glorious weapons over fantastic MIDI hard rock and metal soundtracks.
Enjoy! :)
