But to answer your question before phil gives a better answer...
Imho, I'd wait before investing as current gear (for BTC at least) is overpriced and there is a lot less new gear on market.
However, if you are up for using used gear, there is a whole hell of a lot gear on second-hand market and you might find good deals.
Can you suggest me some second-hand markets,.. is Alibaba good?
Ooof, no, I wouldn't trust any seller on Alibaba, Aliexpress or any market that starts with "Ali" tbh.
I'd recommend you to search local markets first. Like for example, in Serbia we have "KupujemProdajem", Croatia has "Njuskalo" (or something like that and many more), USA has ebay, amazon (not sure about others), not sure what Germany has, but you get my point... Check local market, as there will be no import tax, no big bill on transport, etc.
After that, go to ebay/amazon (but prices are inflated a bit there afaik if they even have miners anymore).
After that, if you really wonna dig deep and find good deals, find RELIABLE telegram groups (this is the worst advice I will ever give as there is so small number of actual sellers and a lot of scammers there).
And lastly, try getting in contact with a friend or someone in China who can personally escort the sale between reliable seller and you.
Not sure what I'd do after that, probably fly to china to buy gear myself (assuming there wasn't a corona problem in 2020).
But what I would do (and am doing right now), is wait to see what Bitmain does with their company and if they release some good deals soon. Mainly looking for Z15 as they look to be solid and earn quite a penny.