You've got your foundation (tank/cleric) already. Where you go from there is basically wherever you want. Class balance here is pretty good. However, there are a few classes which are not as strong as other without donor gear.
Rangers, for instance, aren't very DPS heavy without a few items under their belt (and even after...). Rogues are decent DPS until end tiers, at which point you pretty much *need* donor augs for backstab damage. Wizards are plagued with a few issues. They do nice damage, but that damage is dependent on debuffs from enchanters+druids, plus buffs from bard. Also, your mana regen is going to be a huge issue until T8, and there's always the "BOOM aggro" issue that almost all wizards face. Necromancer/Mage used to be the KING of dps until T8 due to the tradeable t8 pets, but after they got retuned I'm not sure how well they do. They are, however, still decent DPS. Beastlord pet was immune to the changes they recently made to pets, so you can still buy the T8 pet in the bazaar and have it wreck things in lower tiers.
Since you're going to be boxing a full group, bard is a great idea. Their songs are GREAT for boosting group's DPS, and they are great pullers (required in some cases...). Also, a shaman may be welcome as well. They have pretty darn good group buffs, and they are pretty good at patch heals as well. Your tank is going to have trouble staying alive with just 1 healer (cleric) against tough bosses in each tier. Adding the shaman will increase his survivability with heals/debuffs (there ARE a few bosses that are fully slowable) while also adding something to dps (buffs).
Personally, I like the idea of:
SK
Cleric
Bard
Beast
Shaman
Rogue
Edit: Forgot to answer how far you can get

Boxing a full group, you can get pretty high into the raid tiers, especially if you get a donor item or two. For instance: in PoFear (T6) there are several bosses that do periodic death touches. Unless you have multiple tanks, you're going to NEED a decent Divine Intervention buff (donor Ares shield has a 90% chance DI) to mitigate the DT. You can probably get by on chance with the lower end DI (25ish % by that point?) but you're playing Russian roulette, and you're going to need a lot of DPS to prevent that trigger getting pulled against your head too many times.
For progression you're going to face many difficult battles throughout each tier. Some of them:
T1: Emp Srra. Not only is keying *EVERY* toon up to his room a hassle, but the fight itself requires either some very nifty timing/strategy, multiple tanks, or a toon that is geared beyond the zone. Luckily, T1 is an open zone meaning it's not restricted to your guild only, which means getting someone to help you with Emp fight is as easy as an /ooc "I need help with something..."
T2: Vulak. By the time you get to this end-zone dragon you've already learned how to deal with massive AE dots, debuffs, and major damage on the tank. This fight will pretty much boil down to managing these things on a larger scale against a boss with much more HP than the previous dragons you've faced.
T3: Kerafyrm. Similar to Vulak, you're not facing any tactics you haven't already faced, except for the AE debuffs which will easily strip everyone in LoS. You're just doing it on a much larger scale. The main difficulty with the sleeper is his HP. The fight will take you forever to do, and as with all extended fights it REALLY becomes a gear check and your ability to sustain.
T4: Open zone. Most people simply skip it, but if you're going at it alone you're going to want to clear it at least once. The new thing about this zone is the requirement to tradeskill for the armor. This armor will really help you in t5, but there are other armors that I won't get into that you can obtain elsewhere which will help you in t5.
T5: Avatars, then Xegony. You must kill the avatars to reach Xegony which is the end of t5, and let me tell you something about the avatars. They. Hurt. They hit much harder than Xeg does. These are the ultimate gear check for your tank. If he doesn't have enough AC, you lose. You will need to fully focus your attention on your tanks gear in order to survive to Xegony, which is more of a sustained fight (wont go into details.)
After this, the fights start getting pretty intuitive and I don't want to ruin anything for anyone, so I won't describe bottlenecks

But rest assured, if you're going at it with one group, you CAN experience all of the above and much more. But you're going to be spending a LOT of time gearing each toon to the MAXIMUM possibility of each tier in order to progress to the next.
My suggestion is find a new, low-tiered guild (there aren't many, you may have to make one and invite like-minded people) and progress thusly. You'll have a ton of fun
