So far, I've found 2 old looking Thurlby dual output PSUs, a cracked BK precision 1651, a used HP E3630A for 250$, then brands that I don't know anything about: GW instech, Iso tech.
Thurlby Thandar's are decent, as is the BK 1651, and GW Instek as well (i.e.
GPC-3020). Shipping is expensive because they're linear supplies, not SMPS (big heavy transformers & heatsinks). Noticeably lower noise and ripple though, and if possible, I'd suggest getting a linear over an SMPS if at all possible.
Between the GW Instek GPC-3020 and the BK Precision 1651, I'd suggest the GPC-3020 (I know this one has switchable input voltage as well as both CC and CV modes).
I don't want to make anyone angry at me for putting this KA3305P in the same line as an HP E3630A, but would you still buy the hp one for that price?
For that particular model, Yes. They're still in production and go for $698 MSRP. I would suggest getting a unit with CC mode though hence the recommendation above ^ (the linked seller ships to Italy).
What do you think is a reasonable price for a used 3 output linear psu (with constant current mode capability)?
Depends on the market.
Comparatively speaking, they're cheaper in the US than anywhere in the EU/UK as there's just so much more of it available (supply & demand). Since there's so much less of this type of stuff in your market, the prices tend to be a lot higher and the selection more limited.
Importing from the US is a possibility, but you'd have to be sure it's input voltage is switchable on the linear units assuming the shipping rates are feasible as well as any customs duties and fees.
Sorry for going off-topic: I'm also looking for two multimeters, I was thinking of buying the aneng 8009 for 20$ on bangood and 121GW for 280$. Is it better to instead buy 2 identical meters (like the EX330 (60$) or BM235 (105$))?
Multiple meters is generally a good idea as you'll likely encounter the need to take simultaneous measurements. It's also another way to get specific features/functions. Having identical meters gets you the first part, but not the second.
Is there a specific feature in the 121GW you need? If not, you could better spend your funds on say a
BM235 or
BM257, and add in an Aneng and even a Uni-T, say the
UT-139C (no idea what you really need). All for less than the cost of the 121GW.