Is it worth to buy a 30 year old Cessna to build time?

When I did my hour building I towed gliders and dropped parachutists.

Hi.
There is a good saying in the UK "If it Flies, Floats or F***s, rent it". Aircraft ownership is a serious undertaking and it will cost you far more than you could possibly believe. There is no such thing as a cheap aircraft and if you buy one cheap, sooner or later you will pay its true value in maintenance. You need a big wallet for contingencies, like metal in your filter, or a prop strike.
When I did my hour building I towed gliders and dropped parachutists. If you can do something like that you're on to a winner, someone else is paying! Alternatively, many folk get an instructor rating and an airline will look upon that as commitment and an advantage over anyone with similar hours. It's a shame that you only want to get to the airlines, sure the money can be good, but in the early days it's not and there is a fantastic variety of aviation wonderment out in the world to behold, so get some of that and enjoy your flying and life. Another alternative, if you wish to buy, is to set up a group ownership with some like minded people, but make sure there is a legal and binding contract in place so that everyone in the group knows where they stand. Talk to other group owners and find out what they do.
I'm a retired airline Captain and for many years was in airline management. I conducted many interviews for new pilots. I looked most favourable on the low houred pilots who had actually done something. I was saddened by pilots who had only flown 3 aircraft types C152, C172 and PA28 for example. My current tally is 128 different types and what I looked for was experience over just hours in the log book and that is what you need to get, don't confine yourself to one aircraft and fly it round and round. If you do buy, fly it to Alaska or something, make an adventure out of it, it will make you a better pilot for sure.
I also pushed candidates to the top of the pile if they had aircraft engineering experience or qualifications, so go help out in the hangar if you can.
Best of luck with your future, go fly!
Rgs. Bob.

Older planes are usually very good purchases, IF they have been well maintained over the years. Airplanes have to be inspected annually by an A&P mechanic with an Inspection Authorization so they are usually very well maintained. Any purchase should include having an A&P mechanic review the airframe and engine logs and evaluation of the plane, including looking at how many hours the engine has since overhaul and checking compression. I had a 1966 Cherokee 140 for several years and finally sold it when I bought my current Experimental aircraft (a BD-4). It helps a LOT if you can get an A&P license and maintain you own plane though.