I'm working on Atari STe, which has dodgy caps on PSU board. 2 of them are in desperate need for replacement. Can anyone suggest quality brand to extend the life as much as possible?
Don't fix it if it ain't broken
I can't actually see a need to replace these capacitors.
The caps (cylinder ends) are not bulged out and nothing seems to be leaking from them.
The yellowish goo you see between the capacitors is actually intentional to prevent the heavy caps from being ripped off the PCB in case the device receives a mechanical shock.
In case this power supply is working properly: Leave it as it is. If it doesn't, there's no visual hint that this is caused by the capacitors.
Fix it if it is broken
If you absolutely want/need to replace the caps, make sure you obtain them from a reputable source (i.e. not necessarily eBay). Distributors like Farnell or Mouser should supply reasonable quality brands such as Panasonic or Rubycon. This is, however, not an absolute guarantee the caps will hold up another 35 years - in the past, even the quality suppliers had charges that turned out to not hold up for longer than 10 years.
Capacitors are typically ("normal" grade) calculated with a lifespan of about 2.000 hours at 85°C in operation (Thermal aging being the main driver for defects). There are series of caps that comply with high-temperature specifications of 105°C at 2.000 hours or even 105°C at 5.000 hours (e.g. Panasonic FC series). Those should give you more headroom - See if you can get them from a trusted supplier.
As you can see from the specs, the 2000 hours lifespan at 85 degrees C extends to 30000 hours for circuits operating at 65 degrees- the main reason for aging is temperature - also in storage. Keep the computer in a cool place and make sure the power supply is operated under reasonably ventilated and cool conditions. Make sure it's switched off when not needed. Don't bury external PSUs in non-ventilated places.
In general, the advice from tofro is the correct advice.
But if you do have a broken PSU or you're inclined to tinker with a PSU -- which requires some care because it's a thing drawing power from the mains -- there's a goldmine of information for various ST PSUs here: http://www.exxoshost.co.uk/atari/last/psu/index.htm
Exxos also sells capacitor kits for the most common PSU types. I actually did do this replacement on an old and slightly busted machine. I'm not the most seasoned when it comes to soldering, so I got to feel that adrenaline burst on first power-up. Works a charm now, though.