10-year-old Fujitsu-Siemens PC. Change config

Hello,
Beyond about 5 to 6 years, updating a PC is not really possible; there are too many components to change, and there’s a risk of incompatibilities with what we keep. It’s better to replace everything...
If we look more closely:
motherboard: to be changed
this implies changing the processor, the RAM, the graphics card (not the same GPU port), the power supply (likely not compatible), the hard drive (we’ve been using SATA for years)
I’m not even sure you can keep the case, because of the front connectors, and anyway there are decent cases around 35€, like the Xigmatek Asgard
so I advise you to keep your data on your IDE HDD and put it as a second HDD in a new PC
Your budget is a bit low; a basic office configuration (no recent 3D games) is around €360:
https://www.materiel.net/pc-de-bureau/l402/
to which you must add the OS, Windows 7 being around €115

My budget was 350 euros because I didn’t think I would change everything... If I have to redo everything, could I reuse elements from my old one for the new one? If I have to redo everything, the problem is that I don’t think it would be worth it...

If I have to start over, the problem is that I don't think it's profitable.

On the contrary, it's more profitable than trying to reuse old components that risk being incompatible and breaking down, but it's a bit more expensive and it goes over your budget, if that's what you mean.

As for the recoveries, I only see the hard drive for your data and the drive, because on a modern motherboard you can only install 2 IDE devices.

You might be able to recover the case and the power supply, but we know absolutely nothing about them:

- case: dimensions, front panel button connectors
- power supply: photo of the label please, as indicated here:
https://forums.commentcamarche.net/forum/affich-37586385-choisir-son-alimentation-de-pc#mon-alimentation-actuelle-est-elle-suffisante-pour-ma-nouvelle-configuration

The power supply is an ATx-350 model. I'm not bothered by changing the case... The problem is that it isn't for me (I already have a fixed A6117 that would also deserve an upgrade, but normally I should have a portable by now) and I don't think my dad would want to build a PC from scratch... The problem is that a PC rebuilt entirely according to certain criteria will cost, I think, if we choose components piece by piece, 500 to 600 euros to have an excellent PC (versatile, capable of running some recent games), whereas in 5 or 6 months my PC will be fully accessible and a refresh will cost no more than 250 euros. The main idea was to upgrade a PC that sits gathering dust under the desk because it is slow, not to add another unit under the desk. But if that's impossible I don't think I'll go any further...

there are indeed upgrade kits, like here:
https://www.materiel.net/kit-d-evolution-pc/l456/
you should add at least one graphics card such as HD 5750 or GTS 450 for about 100€ if you want to play, we reach 250€
the ATx-350 power supply is probably a 'no name', it may cause problems and you’d better change it: cost about 50€
moreover, you must be sure it fits in your case, and that the front panel connectors (On/Off, Reset, ...) are compatible with the new motherboard
otherwise, it’s another 35€
it's up to you to decide ;)

Thank you, and do you think in the end I’ll have how much? For you, is it profitable?