With the qualifications you provided, I can't see anyreason not to buy it. :D
Years ago I collected old sewing machines in cabinets because I love those old machines. Eventually my dh pointed out that they were taking up a lot of room and had a collection of dust on them! I realized that I had enjoyed fixing up those old machines, but that I would never actually use them because I prefer my Bernina 1230 for daily use. I did sell that collection off to another collector. (I still have several old portables that I need to finish fixing up and sell them.)
As long as you are pretty sure you are going to use the machine, I say go ahead and get it! Try it out and make sure you like it that much before getting rid of your other machines. If it doesn't work out for you, you can always sell it.
Just try to purchase it for a reasonable price so you can resell it for a reasonable price. If it's worth the trouble to you to resell and you figure you're only out $20 or so to test out your heart's desire, I say go for it! The Singer 401A is definitely in demand as a collector's sewing machine, so it will retain value over time. My concern, though, is that sometimes these "antique" machines are overpriced for sale. Machines and cabinets have to be in top condition to get top price. Could you post the price and maybe a pic?