Because you don’t have much dust, personally I’d suggest first using a few of those closer to 2500 and trying to build a team to counter those more frequent UL opponents such as Giratina-A, Togekiss, Swampert, etc, and later on to power up some more powerful but also more expensive pokemon such as Clefable, Steelix and Snorlax.
For example, you have a Sceptile with Furry Cutter + Leaf Blade, which is very solid against Swampert as long as you don’t get tricked to let a Sludge Wave through. It’s also a very spammy combination which can be quite handy. Dragon Claw looks tempting, but Sceptile won’t be able to beat Giratina-A no matter what because of lack of bulk. If you have a charge TM to spare, you can consider change it to Earthquake to hit Steel and Poison, or Aerial Ace to surprise fellow Grass such as Venusaur and Torterra, or some Fighters like Machamp, depending on the rest of your team. Or if you see that your final team doesn’t have a strong Giratina-A counter, doing chip damage with Dragon Claw is also worth considering;
You also have an Aggron. With Dragon Tail + Stone Edge + Heavy Slam, it usually loses to Giratina-A with Shadow Claw with relatively close margin, and can win against Giratina-A with Dragon Breath (generally less frequent) with also a small margin. So with correct switch strategy and the help of your other two pokemon in your team, it’s possible to take down Giratina-A. Moreover, Aggron resists to Togekiss and Clefable’s fast move Charm, and can hit back with Super Effective Heavy Slam or Stone Edge, so it can also serve to check Fairies (but beware of Granbull’s Close Combat which can annihilate Aggron). You can also consider changing Dragon Tail into Smack Down, which allows you to hit Fire and Flying much harder, both of which can give trouble to Sceptile, but you gain energy much slower, which makes you lose harder to Giratina-A especially if it still has a shield.
Now, you have your Aggron which can check Fairies such as Togekiss and Clefable, as well as a fighting chance against Giratina-A, but it’s weak to Fighting, Ground and Water, and have no answer to Steel for example. Ground and Water can usually be checked by Sceptile, but you’ll need to choose whether to have extra coverage for Fighting and Grass (Aerial Ace) or Steel and Poison (Earthquake) depending on which type you see more often, or to do chip damage to Gira-A with Dragon Claw. So for the third mon, I’m inclined to choose Dragonite, and changing its move into Dragon Breath + Dragon Claw. Ideally I’d suggest unlocking its 2nd move and preserving Outrage for a nuke option, but candies and dust can be costly. With DB + DC, you have the possibility to narrowly win against Giratina-A, as well as wall Swampert if it has its Water + Ground moveset. This way, you don’t need to rely on Sceptile for chip damage, thus freeing it for other coverages.
So now, you have a team that can more or less cover each other. Dragonite has generally good matchups against many things except for Fairy and Ice, and can wall Swampert. Aggron can help you against Fairy and lend a hand with Giratina-A, but is weak to Swampert and such, which can be checked by Sceptile, which itself is not effective against Fire and Flying which can be somewhat checked by Aggron and Dragonite. Of course, there are still many glaring weaknesses, such as Ice, which both Sceptile and Dragonite are weak against. Aggron can’t really handle those Water type pokemon with Ice move as coverage, while Sceptile will need to be much more careful. But at least with this approach you can more or less have a budget team as a starting point to work a strategy around it, such as which pokemon to lead in the battle, how to lure out the big threat of your team, etc, without breaking the bank.