For riding a few miles ad day, just use what you have and cobble something together. For the century ride, absolutely look for something more modern, and buy used. Get the right frame size and then have the bike fitted by a bike fitter.
I did just that when I had a similar plan and ended up doing the "Death Ride" in 2018. I bought a top of the line used Colnago C50 with Campagnolo group off CL for not a whole lot of money. I put modern saddle and handlebar at the time I got the bike fitted. I completed the event and had a ton of people comment on the bike. Modern classic. I have been riding mostly MTB since, but it is a great wall ornament in the garage and I can sell it anytime for more than what I paid and put in it.
If the ride is pretty flat, anything 15 years or younger with rim brakes should be pretty affordable. It looks like everyone is getting disc bikes now.
Here my thread. Longest thread I ever started, I think!
- a lot of good info, you may want to read it. Some of the PPOT biking and exercising legends on there. Including Angela. RIP.
http://forums.pelicanparts.com/off-topic-discussions/910181-school-me-road-bikes-11.html
Cheers,
George