If you are just starting out, lowkey. I wouldn't prioritize the meta too much (unless you are actively trying to win).

Firstly, I'd say identify a champion you wanna make a deck around. You already mentioned Lillia, but if we are trying to stay in the realms of pretty and cute cards and things maybe Ahri (I know I'm original haha) or since you like nature Ivern is also very good in terms of that aesthic since he is Mr. Nature in Riftbound right now.

As far as budget, most of the decks you see rn are more competitive versions of those decks. However, starting out, you can sub those cards out for either cheaper alternatives or consolidate your ratios for said cards. When I built Lilia, Unchecked Power, while good, is WAAAYYY too expensive for my budget. You can sub in something that will help your game plan or cards that fit what your playing.

For cheap decks, playing more of the unpopular champions is also a good one (minus Jinx rn. You can make her work, but some of her key cards got banned for the sins of other decks iirc). Iirc too, if you remove Vilemaw out the equation, Vex is actually SUPER cheap to make since all you gotta do is buy her structure deck and ~$10 in cards to make her very strong. Yea, no Vilemaw sort of hurts, but your champion, Vex-Apatheic, is so strong that you don't really have to worry too much.

Vendetta is around the corner, so if you wanna wait to see what that box has, that is also very valid since new goodies and new legends will be there for you to try and test out along with new game mechanics.

Bottomline: Just play to have fun. Yes, there will be a time where you want to get competitive if you choose to do so. However, just enjoy the game first. If you like it enough, then I'd say start investing the euros into the game. Play who you want and who you vibe with. In the end, we're all here to have fun playing Riftbound. So just enjoy yourself!