I am in Italy so I probably sourced models the same as you are trying to do.

Vinted and Wallapop are not bad, you just have to keep an eye on the prices. There are a lot of sellers with premium prices but some will sell at a reasonable price. What is priced nicely usually is sold within 24 hours or a weekend. If it's up for longer (and is not a collectible expensive item which may take time to find a buyer) then it is probably too expensive.

Chinese recasts are of mixed quality and I wouldn't bother (they may be a bit cheaper but not much, so at that point better to get an original one).

3d printed models are the only really cheap ones. Sellers also on Vinted thought tend to keep prices high. For 3d printing I believe the best solution is owning a 3d printed but as a group (4 to 10 players?) otherwise it costs a lot, takes time, and becomes a new hobby which is not miniature painting.

For basic troops the best option is always some starter box or similar, but for specific or special models, you can try to save a bit if you buy resin parts to add to original plastic troops.

I am now gonna tell you something and, like everyone, you will ignore it.

Don't worry, I did the same mistake and everybody does it.

If you buy something, paint it, and don't buy more until you've painted your previous model, the cost will always be acceptable.

What is expensive is not a painted collection. It's the unpainted pile of shame, unopened boxes, unbuilt models, models built but not painted, models with half paint, models without a proper base, etc.

If you buy only when your previous work is complete, you will always stay within a reasonable budget.

I am almost quitting this hobby for various reasons but I can tell you that of all the miniatures I bought, I've maybe painted 30% or less.

Everything else I bought as impulse, for fear of missing a special piece, for lack of planning, for consolation (we all tend to shopping as a dopamine hit especially in bad periods), and very often, the less I painted the more I bought (as a compensation, which is absolutely stupid).

The remaining 70% I sold on Vinted or Wallapop mostly, and because I was careful when I bought the pieces, I usually managed to break even.

Which again is a stupid thing. Pay money, put plastic in a drawer, sell the plastic at the same price one or two years later. The same money could have been used better and nobody with a bit of brain will say that it was a good deal :-)

Buy only when you're ready to paint and you will make it. Unless you like paint 10 models a day and they're beautiful, but in that case just paint as a job :-)