classic volvo fisr time start in 20+ years

20 years, an awful long time. First thing, all the fluids are ancient, and have probably broken down. If the car is decent and you plan on running it yourself drain everything before you attempt to start it.

Drain the gas from the tank. All Volvos have a large drain plug on the bottom of the tank. By the condition/smell of the gas, you'll get an idea of what's in the carb (s). You may want to add some fuel cleaner to the fuel you add since there may be gum/varnish/crud in the rest of the system. Put some oil in the carb dashpots.

Make sure there's some fluid in the rad. Wouldn't bother changing that until you know the rest is OK.

You must drain the oil, fill it with cheap 5w30, remove the spark plugs and crank it over until you've got oil pressure. This is a must if you don't want to ruin the engine.

Will it start? If the stars are aligned in your favour, yes. Make sure you've got spark and fuel. Ignition points and condensers usually fail first. Fuel pumps can be difficult. Clutch or pilot bearing can seize so make sure it's in neutral.

Lots of stuff to check. Volvos are generally overbuilt with quality products. Go through things methodically and find a knowledgable friend to keep you from straying too far off target. Post some pix of your find.

if you wanted to you could pull out the distributor and put a drill on the oil pump for a while , you just need to make a shaft that looks like the one on the bottom of the distributor to put in the drill, and turn the drill in reverse.

but if you've already been cranking it it probably pushed some oil into the journals anyway. Doubt if it is rusty inside the engine if it had a hood over it.

change the filter and oil for sure first. and again after it starts and runs for not very long.

I'd pull off the covers on the float bowls and clean them out. and when you start it make sure the float isn't stuck and overfilling or you could have a fire. you are going to want to clean the pistons in the SU's but I'd see if it runs before doing that. you could pull the air filters off , probably need new ones anyway, then lift the piston upand let it fall make sure it isn't frozen up. it should go down slowly, and you can top up the dashpot things with ATF ( black knobs on carbs)

wouldn't hurt to take the rocker cover off and douse it all with oil when you fill it.
you are going to need new brake parts and clutch parts. Use silicone stuff for the clutch if you change all the seals in master and slave.

I'd rig up one of those plastic inline filters for the gas line, so you can see if you are getting crud from the tank.

squirt some oil in the plug holes and crank it over lots, the rings might be stuck so take it easy on it for the first while.

It'll run , it's a Volvo

Phil

Often as not, '60/'70s Volvos were abandoned over simple parts problems:
1-astronomical dealer prices, and 2-no knowledge of aftermarket parts sources, although parts were dirt cheap at the aftermarket dealers. This means that chances are fair that the car can start.

Check the tailpipe for oil scum: if there is none the engine should turn manually. If it does, bathe carb pistons and ALL LINKAGES AND BRAKE MOTION POINTS with a true industrial penetrating oil (Kroil, Zep, NutsOff). Give that hours or days and repeats; not just minutes. Grasp-wiggle-feel & watch outer pivots of throttle shafts, testing for slack in throttle shaft bearings: there will be some on a 200K mile car but if it's much over 0.20 it could be tough to drive the same day. Check if linkage strikes both carbs simultaneously or close. Top off all fluids as other repliers advise.

Common faults I saw in old 122s: shot front (rubber spools) control arm bushings due to wrong (American action) shocks: shot driveshaft (rubber donuts) carrier bearing cushions: stiff hydraulic clutch master cylinder. If the engine will turn, and if you proceed gently and it's an old Volvo, it will start and run.
Good manuals to have are Haynes or Clymer: others may throw curves at you on the B18/20 engines. Used and new parts and good advice at Voluparts in Atlanta, GA.