I mowed with (and fought with) a 50's Dearborn 5' sickle bar mower for several years. Bought it at a farm auction for $35. It worked, but it took a lot of maintenance. Shook itself to death, broke pitmans, problems with bearings, etc.
This year I finally gave up, gave the thing to a neighbor for parts for his, and bought a used 7' JD 39 for $700. I can't believe how much better it works and how much less tlc it needs, at least so far.
But a sickle bar mower is always going to be a high maintenance item compared to a rotary cutter. Regular sharpenings, and adjustments to keep the sickles next to the guards, are to be expected. Periodic replacement of tooth guards and sickle sections and the occasional pitman stick is also a fact of life. I only mess with the thing because a rotary cutter is a poor choice for hay.
If you go for a new one, make sure parts are readily available, that's very important with these things.
If you go for a used one, take a close look at the straightness of the bar. If it's bent, you'll be fighting the thing to get it to work right.
I saw multiple JD 9's and JD 39's for sale around here (CO) for $500 to $1000.
There's also some Italian stuff on the market, new. I shied away from them over the parts thing.