Your existing crankset likely will not allow for a granny ring. Head to a bike shop and ask them how small of chain rings you can install on your crankset. My guess your current crankset cannot accommodate a much smaller ring and certainly not a granny ring.
Bike shops can look at your bike in person, make informed decisions, and provide help to do the work. They have a huge advantage over anyone on the internet.
Cheapest route? Find a used bike with the gearing you prefer. I routinely see nice used touring bikes out there at reasonable prices.
Maximum flexibility for all terrain = triple crankset, which will mean new crankset, new bottom bracket, new rear derailleur, and more.... a long list of stuff typically way in excess of your budget. A compact crankset does not provide maximum flexibility but reduces the scope and cost of the project significantly (will not fit within your budget either).
Buying parts on the internet is best suited for someone who knows exactly what they want. A local bike shop will cost more, but they take out the hassle of making mistakes, buying the wrong stuff, and so on. Depending on where you live and local bike shop labor rates, the labor alone will exceed your budget.
As far as ancient bikes go, my newest bike is 1992, my oldest is 1973, with a lot of stuff inbetween. My main rider is a 1987, but it has upgraded drivetrain.
Last edited by wrk101; 03-29-22 at 11:07 PM.