Just a couple of things to add to Luke's excellent advice above.
When buying the radial seals try to obtain OEM or at least the same design as OEM. There are two different sorts of radial oil seal which require different approaches to fitting as described here.
https://www.skandix.de/en/news/oil-s...ferences/1374/
The 940 uses type A.
Whilst you can in a pinch knock the new seals into place with an improvised tool, IMHO there is no substitute for the genuine Volvo assembly sleeves.
Petrol and a stiff brush is effective at cleaning the gunk off the front of the engine once all the belts and pulleys are out of the way.
When assessing whether to follow Luke's advice about replacing the timing cover gasket bear in mind that if you do damage the sump seal removing and refitting the sump is a non-trivial task on these cars. The same goes for if you inadvertently displace the PVC oil return tube. So take care and don't force anything.
Good luck!