Help with team and moves

Easy investments would be in the starters. Buy Earthquake for Meganium, Ice Beam for Blastoise, and consider getting a Raichu instead of Jolteon. Cheaper to buy the second move, and while brick break isn't great, it's a fighting move, which can be very useful. Jolteon is stuck with electrics, so no subtlety or surprise, and it's not really worth buying the extra move unless you have Last Resort.

Ultra league team sounds solid, buy the poison move for FP Venusaur. Flygon is a beast, but is very specialized, maybe best not to lead off with it. Lapras is good, but expensive - Walrein is a good cheaper option, although without Ice Beam.

Have any BB Charizard? Buying DC is cheap, and very useful - great neutral damage if nothing else.

We're still learning about what's good and what's only fair. Have fun exploring it with all of us.

I'm not sure what resources you have available to you, but one of the most powerful teams I have in the ultra league (with a very high win percentage) is the following:
• Giratina - dragon breath, shadow sneak, dragon claw.
• Lapras - ice shard (legacy), ice beam (legacy), surf.
• Cresselia - psycho cut, aurora beam, moonblast.

Giratina is the opener here, with dragon breath putting out strong DPS. The charge moves both have low energy costs, so they become available quickly. With Giratina's bulk, you can take hits without usually needing to burn shields, while your constant barrage of charge attacks will often cause your opponent to use up their shields.

Lapras is there to help bring down dragons and anything that flies. It's also pretty tanky, but you may have to use a shield to keep her up. Surf charges up much faster than ice beam, so it's useful if you need to keep the pressure on more regularly (or if your opponent is weak to water).
*Personal note: Lapras with non-legacy ice moves (Frost breath and blizzard) is not as threatening.

Cresselia is the last out, typically, and functions as the cleanup crew. She's ridiculously tanky, and can sustain a ton of punishment before going down. Psycho cut has low DPS, but very good EPS, allowing you to use charge moves with good frequency (often out pacing your opponent). The two moves offer good coverage and, at this point, your opponent probably has no shields left. If you still have a shield, then you've pretty much got this in the bag. If not, Cresselia can still likely tank her way to victory.

That said, there is one flaw with this team: it isn't exactly all that fun to play against. I haven't been using it as much for this reason. In a few test runs, I told some friends exactly what my lineup was and told them to bring specific Mons to counter them. Even in those instances, they found the team difficult to deal with. But if you want a team that consistently performs well and can be brought to just about any matchup, this is the one that I would personally recommend.