No problem in that case providing none of the paint is flaking or loose you should be able to use a specialist primer.

Obviously the paint isn't going to make everything smooth so it's worth sanding down any lumps and bumps but the primer eliminates the need for sanding every surface, multiple times.

The primer is called bin sealer and it's really personal preference whether you use the water based version or the shellac version. I personally don't feel that positive towards the water based version and swear by shellac which is usually more expensive, dries in about 15 minutes and blocks all stains, knots and pretty much every decorating problem known to man. The downside is because it's shellac if it's knocked it can chip but my argument is if it's knocked your
Going to have to fill it anyway so makes no difference.

I'm not sure on the amount of items you need to paint or even the colour - the darker meaning you need more coats but I would say

2.5l zinnser bin sealer
2.5l top coat

Is a very good starting point.

Start from one corner in the room,
Paint everything in the sealed and by the time you get to the other side your probably find it's dry enough for a 2nd coat.

Then apply your top coat. Depending on your plan you may well need to apply a 2nd top coat but it doesn't need that very often so I doubt it.

Sand problem areas
Hoover
Apply 1 bin sealer
Check all painted surfaces for problems
Apply 2nd bin sealer
Check for problems
Apply gloss/satinwood