PC ATX Powersupply modifications

[Delta]

I use Antec TP-550 power supplies with digital recorders that are worth 5000$ each so you can go with this brand without worries.

And maybe this link will help getting some answers: http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_Power_supplies/amiga_power_supplies.html#Fitting_a_new_PSU_to_an_A4000

[Delta]

I remember making the conversion from scratch on my 4000 long ago but it was an AT power supply, not ATX.  Never the less I used a voltmeter and checked every single wire BEFORE plugging anything.  

Make sure you get the correct voltage on each prior to connect, that's the best advice I can give.

[Delta]

Yes, voltage in power supplies is never perfect unless it is for sensitive electronics.

You can trust these values, its all normal.