upgrading components on a Brava late 80 early 90
So many changes - unless you're particularly attached to that bike, consider a new one with the components you want.

Otherwise your shopping list is new shifters, new cassette/freewheel, new chain.

You probably need to check the new chain will run on the existing front chainrings, else they need replacing.
If the new cassette/freewheel fits the existing wheel then that's good, otherwise its a new hub, possibly new spokes and a wheel rebuild, or a complete replacement wheel.

The rear deraileruerueuruer is probably OK, they are not indexed and don't really care.   However if the wire pull is different between gears, this needs changing too.

The front one is likely OK, unless you want the same type of shifter lever front and rear.

Just remember, a new bike will be lighter and have a warranty, and you could choose one with disk brakes if that appeals.