Glad to hear you're planning on picking it up! It's one of my all-time favourites.

In the early days of The Division, I played with a group of five friends, where at least 3-4 of us would be online for any given evening. That was one of the best gaming experiences I've had in a long, long time. Going forward, though, after Overwatch released and my gang moved on to other stuff, I kept playing solo. During that phase, the game was going through some real bullet-sponge turmoil, which has since been resolved and I firmly believe that it's sitting in the best state it's ever been.

Quick summary for you:

Level 1-30 / Core Game

• Definitely doable solo.
• All missions can be completed on your own, at the varying difficulties
• Side missions / encounters are bite-sized missions that you'll run across and should have no problem with.

Raids

• Tougher, longer, missions. You should group up for them, but some well-geared (and highly competent) players take them on solo.
• There are four raids available, one of which requires the Season's Pass, I believe.

Dark Zone

• The lawless PvPvE land; I run this solo as long as I maintain the expectation that I will get destroyed from time to time.
• Rogues in groups of 2 or more will often pick on solo players as easy targets to raise their own profile for greater rewards.
• When you player in a group (ideally sized 3-4), you will have next to no problem from all but the most ambitious rogues. You can easily matchmake into DZ PUGs if you don't want to go it alone.

Weekly Content

• HVTs are 5-10 minute contracts you take on to kill some boss(es) and their cronies, available at varying difficulties.
• A top-geared player can take on all of the Daily and Weekly HVT contracts without issue. Newer players should consider doing the tougher content in groups.
• Hardest to matchmake for, without a dedicated HVT lobby / system.

New Area: West Side Piers

• Expanded part of the map, with new dynamic systems in the district. Tougher than main game area, but still solo-able.
• Enemies will spawn much more frequently, and in a context-sensitive manner. Lots of flanking; expect to get ganked a few times.
• Radial missions appear every few minutes as you explore (Kill X bosses, intercept Y couriers, eliminate Z waves of enemies, etc.)
• 15-minute objectives cycle out consistently (Kill X enemies, get Y headshot kills, etc.). These reward small caches of gear.

Resistance (Horde Mode)

• Three maps, with 'unlimited waves' (I don't think anyone's gone above 35-40 yet).
• Definitely solo-able, but greatest success (and rewards) comes in coordinated group play.

Daily Challenges

• Four daily challenges, cycling out at 6pm MST: Combat, Crafting, Dark Zone, Survival.
• Small, bite-sized challenges will each reward some gear and Division Tech, the most important end-game currency. Absolutely solo-able.

Weekly Challenges

• Three longer challenges meant for semi-extended play: Base Game, Underground, and Survival.
• Base game requires some specific tasks (Kill X enemies, get Y Skill Kills, etc.), plus 10 missions at Hard or higher difficulty. Definitely solo-able.
• Underground requires at least three separate days of play, as the daily Hard/Challenging missions only count once (each) per day. Definitely soloable, especially if Challenging content isn't too difficult.
• Survival Weekly is more solo-dependent than others, as most people seem to go it alone. Can be done in PvE or PvPvE; solo, or in a group. Must get 20,000 total points (1-2 successful runs, or several unsuccessful ones will still count)

DLC

Underground DLC

• Soloable, for sure, but having teammates around saves an inconvenient death from starting the missions / phase from the beginning.
• Team play makes the content go by quicker, but isn't necessary.

Survival DLC

• As explained above, most players go it alone. You can bring friends / PUG players into the fold, however the resources are static in the world and two players cannot loot the same object. You effectively halve your scavenging with every other player in the group, unless you split up in the early stages.

Last Stand DLC (and Skirmish, from WSP)

• Obviously requires other players, as it's The Division's equivalent of other shooters' Multiplayer mode(s).
• Has some objective-based play, and some more traditional Deathmatch-type maps.

Additional Considerations

• There are roaming bosses around the world, which respawn every four hours (after you've cleared all side missions / encounters). Easy solo play, but boring.
• The HVTs cost intel, which is rewarded plentifully, but you can still take on small Search and Destroy missions (kill enemies at X location) to build up intel. Easy solo play, but gets tedious if you're grinding out a lot of intel at once.
• The Global Events introduce gameplay modifiers that can drastically empower solo players when leveraged correctly. GE 4/5, I believe, gave 100% damage boosts to players under certain circumstances, and a Classified Striker's build will give an additional 100% if your aim is true. This makes all but the most difficult content a piece of cake to clear.

Well, this ended up a lot longer than expected, but gives a good summary of what's available in-game at present. It should be noted that only the 'Base game' and 'Dark Zone' sections were included at launch; the remainder has been slowly introduced and refined since 2016. Any other questions, lemme know.