You could switch to 8-speed index shifting fairly inexpensively if you shop for the parts.

Since your bike is 6-speeds (125/126), the rear drops are already wide enough to use a 8/9/10 speed 130mm rear wheel (130mm). You probably just need to exert and little outward pressure on the dropouts when installing the rear wheel.

Look on Craigslist of a set of 700C 8/9 cassette wheels. You might even find one with a 8-speed cassette still on it. I've bought several sets of 8/9 cassette wheels on CL this past few months for between $10 (brand-new Nashbar wheels in the box) - $20 (used Vuelta Airline wheels) a set. The average price on CL is $50-75 a set if you don't want to wait for a deal.

When switching from 27 1/4 wheels to 700C wheels, you may need to replace the brake calipers with ones that have a long reach (700C wheels are smaller diameter). I would recommend modern Shimano dual pivot brake calipers if you need to get long reach brake calipers.

8-speed components do not cost more than 7-speed components and are just as durable, if you are changing up from 6-speeds.

A new Sunrace Shimano compatible 8-speed cassette is $17 shipped on eBay. Used Shimano 8-speed cassettes are about $10-$20, and new ones are $15-$30

A new set of Sunrace 8-speed index shift levers with a clamp for downtube or stem mount is less than $25 on eBay. I just got a set of Shimano SL-R400 8-speed braze-on shifters for $20 from a eBay Seller.

I got a used working Shimano 105 rear derailleur with long cage off CL for $15.

New KMC X8.93 chain was $12.50 off eBay, but you could get a KMC Z72 chain for $9.50.

So, you can probably do the upgrade to 8-speed index shifting for less than $100. You can sell the old parts if they are still serviceable to defray the cost of the upgrade, OR save the old parts to put back on the bike when you are ready to sell it, and take the newer parts with you.