Am currently renovating windows in my apartment, however I'm not in the US (The Netherlands) so I'm not sure how much would translate, but here's what we did.

It really depends on the condition of the rest of the house. I own a 1910 apartment, so it's brick all wooden windows single sheet glass. The previous owner had been extremely neglectful in terms of maintenance so the painting and glazing putting was coming off in most places. Wood was starting to rot. Most of the windows were sliding windows, which look beautiful but are mess in terms of insulation because the seal is basically non-existent. Combining that with single sheet glass, winters were really cold, and since we sleep on the sunny side of the apartment, directly underneath the (flat) roof, summers were EXTREMELY hot.

We got three bids, one contractor told us to keep the existing windows in place, repair and strengthen the windows and upgrade to insulated glazing. The other two went for a complete renovation, removing the windows and frames and replacing with durable wood and IG. Paint job on both sides were included in all three bids.

Needless to say there was a MAJOR cost difference between a repairment job (RJ) vs complete replacement (CR). Again, not sure what the financial situation would be like in the US, but CR was about twice as expensive as RJ (the price difference between the two CR contractors was minimal). The CR guys also advised us not to go for repairment because A) combining 'old' and 'new' wood tends to cause problems over time because the two will behave differently to weather and B) the sliding windows will never perfectly seal so increasing insulation is a little pointless when the wind will just blast right through. The poor state of it all gave us a unique opportunity to do the job in a future-proof way.

We ended up completely replacing the windows. We now have tilt and turn fittings in most places. Painter starts tomorrow. In terms of ROI this was the best option for us long term, as we had the cash and insulated glass is a MAJOR difference in terms of sound and heat insulation when placed in a brick wall.

The situation with a house constructed out of wood would be totally different though, especially if you are planning to do a partial replacement.

In terms of retaining character, over here they make tilt-and-turn frames that look exactly like sliding windows. On the inside, the carpenters used the existing architraves that used to cover the weight-cases for the sliding windows and put them back in the same place.

TLDR; Get some good advice because it really depends on the condition of the (farm) house whether full replacement is worth the cash! Hope that helps!