Hi kasper21,
Last night I took a look to all the 4 systems and the kits mentioned by you along this thread(on olx, I'm from RO also...) and I would do like this:
1. I would definitely buy the P166 MMX system, the case is very nice and is in very good condition, probably you could max out the motherboard's potential with a P233 MMX or even an AMD K6 @300MHz... it's a socket 7 mb, not socket super 7, but...
2. ... for a somewhat faster system I would go for the PII 333 MHz, because it's a HP Vectra VE system with standard ATX motherboard, standard ATX power supply, basically it's a standard layout desktop PC. The motherboard in this system is a FIC VB-609 with Intel's BX440 chipset(the best for a PII system), but it has only 100MHz bus. Anytime you can put a PIII 500 MHz on it if you want a little more speed. I don't know if you can go higher with the processor, if I remember right, this mobo has only a 5x multiplier. These HP Vectra systems are really-really reliable.
3. The Celeron 400 system is a little bit odd, from the BIOS string and the pictures from the ad I can guess that is a Soltek Slot 1 mb with a VIA Apollo Pro Plus chipset. It's odd, because it is an AT form factor slot 1 mobo. If I'm right with the chipset, it has only 100 MHz bus, so the upgrade path is not that wide. If you are not a retro PC collector/enthusiast, you should go with the HP Vectra system. If you like to collect rare/odd systems, you should go with this system. If you want to play with old software, you should definitely go with the HP Vectra. The CPU is on a slotket adapter, probably an older one, but it's a nice piece of hardware...
4. In my opinion, you should definitely avoid buying systems like the PIII 500 Fujitsu, because it has a proprietary(rare) motherboard/PSU/etc. format! If something brakes in these kind of systems(mb, PSU), probably you can throw out the whole system because you probably won't find the replacement part. Again, if you are a retro PC collector, it's an interesting system to have, bot not for 45 euros 😀, maybe 5 or 10.
Regarding the kits, actually any mb+CPU+RAM kit, you should take in account that you will need to by the rest of the system... and you could end up paying a lot more to have a complete PC.
As for the prices, if you are not in a hurry and have some hardware knowledge and willingness to clean up dirty hardware 😀, you could find this kind of systems on your local flea market for 10 to 15 euros, easy. I don't know where you live, but in CJ I bought this week a socket 7 system with an AMD K5-PR100 CPU in a nice AT case with 10 euros and a beer 😀(50 ron), last month I bought a P100 + 24 Mb RAM + 1 Mb VGA + 1,2 Gb HDD + Creative CT2900 sound card + FDD + CDROM + a nice AT case amd PSU for the same price. For 1 euro(5 ron) I got a P166 MMX CPU + heatsink... Most of the sellers don't even know what are selling... Anyway, if this "activity" is not for anybody 😀
So, good luck! Hope my post would be at you help...