Put a freewheel on the back with the largest big cog that your rear derailleur will handle. Put a compact double with the smallest rings (both rings, e.g. 48-30) that you can find. That's an effective and inexpensive solution. If you plan to ride up a lot of very steep hills than you may have to go to even lower gear ratios which would mean a triple chainwheel and a new rear derailleur.

Bar end friction shifters would be an additional expense but the bikes original downtube shifters are easy to adapt to. Just remember to shift before you're in extremis. Try using them for a year.

Brake/Shifters (brifters) are very expensive and will complicate your setup.