oh wow. How can you even play on that!
Any class in particular you're looking for? It might be hard to give advice that's super general for all classes. Are you building the character with point buy, or standard array of stats, or rolling for it?
In general, the D&D players' handbook (or, I guess, the video game?) when describing the class tells you what the most important stat for that class is. E.g. Intelligence for wizards, Wisdom for druids, dexterity for rogues. Some classes rely on two main stats; e.g. barbarian probably needs STR and CON, fighter can either do CON and STR or CON and DEX. Pick a race to go with the class, one that increases the stats you care about by 1 or 2. Make sure those stats are decently high to start. E.g. after racial bonuses, your top 2 most important stats should be at least 16 and 14 if using standard array.
If you're optimizing with point buy instead of using standard array, it's possible to do better; I think with point buy you could either get a starting stat to 18 (after racial bonuses) or get two starting stats up to 16, at the expense of having more dump stats at -1 or +0.
If you're looking for a safe build, don't multiclass. You can't really go too wrong by going single-class and then picking a subclass, there shouldn't be any "trap" choices.
Other than that, lots of things are class-dependent. If you've got just one primary stat you care about, later on in levels it's important to get it up to 20 ASAP. Other characters are more flexible and should detour for some feats. Etc.
5e really feels pretty idiot-proof to me, if you do the obvious things you'll end up with a pretty good character IMO.
OK Yeah, I'd probably keep that map as a digital file, and put big Xs on it when the PCs have cleared a room. I'd probably keep a separate doc with the rooms the PCs have visited, listed out with any notes about what happened there.
Since there's a detailed map, I'd probably never change around the layout of the rooms, that would just be annoying to keep track of. However, I'd feel free to change what's in them in terms of treasure and enemies, depending on what made sense at the time. Note that in the doc as you go.
If you're feeling ambitious or are familiar with an image editing program, you could have a PC version of the map, which you send them after each session, removing the fog of war from the parts they haven't explored yet. They probably need to see the map that they've already explored, to stay oriented.
I probably would read ahead for the ring of rooms nearest where the PCs are. They can always take a totally different tack, but might as well be prepared for the most likely one.
Oh, wow. 240 rooms is a lot. Are they at least spread over multiple levels?
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Even in a dungeon crawl, you can improvise a lot. The contents of the rooms, spells and secrete doors, treasures. I don't know that specific dungeon, but I've never had a D&D game, including a dungeon crawl, where everything went as the module planned.
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It's useful to read ahead if you can, the you can seed hints of things elsewhere in the dungeon.
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A DM-only copy of the map is really useful, so you can make any notes you want on it. Paired with the list of rooms and what's in them - that's where I'd note down for myself what the players did in each room, if there was anything interesting not note down besides "cleared".
All the many times that I as DM gave the PCs an encounter, and they found a way around it without fighting. Those are always fun to watch and see what players come up with!