YMMV...as always... Please bear in mind that I only shave my head, so my experience is bound to be different than the average face-shaving experience.
My favorite vintage DE is my Super Speed. It is the notched version, with no date code, so it was made somewhere between early 1948 and late 1950, but that is as close as it gets for a guesstimate. Very mild, but very efficient for my head shaves. Provides a very comfortable shave that can be completed "brainless", meaning that the mild nature and blade exposure allow me to be fairly "lazy" in it's use. That merely means I don't need to pay as much attention to my angles, as there is simply very little risk of nicks/cuts. Price range is anywhere from about $20 to about $40 depending on condition.
Second favorite would be my 1963 Slim adjustable. I like it from 2-6, depending on my stubble. I find it to be very comfortable, very efficient, and the adjustability makes it an excellent daily driver as it will fit a variety of situations. $30-50, dependant on condition.
Third would be my Old Type set. No date code, but it has a thick top cap, so it is somewhere between 1921 and 1928. Without any patent markings or date codes, that is as close as can be had for a guesstimate. These guys had pressure-fit handles, and as such are notorious for cracks. Pay attention to the center tube of the handle where the end fits into place, and the screw post receptacle fits in. This is where you will almost always find cracks. Many of these cracked handles can be stabilized, some are worthless, but the head will still provide an exceptionally close shave, albeit a bit aggressive for my taste.(I have a Brownie Set that is in outstanding condition, including the case and blade box. PM if interested in trying to strike a deal). $20-100 dependant on condition, boxed set, rarity, handle type, etc...
I have a GEM G-bar which is a great little shaver. Circa 1940-50, these little guys are fairly common still, and can be found for a very reasonable price in most places. Good little SE. $5-20 dependant on condition
I have used a Tech and found it very similar to my Super Speed. I have also used a FatBoy, and found it less enjoyable than my Slim, for very similar reasons to what
Freddy outlined above. I have no room in my modest den for repetition and redundancy, so I no longer own those razors.
Be aware of pitting and plate loss. Some is tolerable, but too much can be troublesome, unless you calculate the additional expense of replating.
If you're looking at adjustables, make sure they adjust through the full range. Make sure the adjustment plate actually moves up and down, not just turning the dial with no movement. Make sure there is no gap between the top of the adjustment ring and the bottom of the top plate. A gap at the top of the adjustment ring will typically indicate an adjustable that has been "overclocked" for more aggressive blade exposure, and will require service or repairs to be properly functional. Make sure they click and lock into place.
With any TTO, make sure the barn doors open fluidly, set securely, hold the blade tightly, and do not close "off center". The doors need to be aligned for proper operation, so try to pay attention to that in your searches. If possible, put a blade in there, and make sure it doesn't move around and aligns correctly when the doors are closed.
All of these vintage issues are repairable, for the most part, so if you are willing to spend a few extra bucks after the purchase to get something fixed or replated, no worries.
All of the price ranges I listed are highly subjective and variable. You will find razors both cheaper and higher than the ranges I've listed, so look around, look CLOSELY at all pictures, and most importantly...ask questions of the seller. If the seller is unwilling to provide more pictures or answer specific questions, I would suggest looking elsewhere. A trustworthy seller should not have problems answering questions and providing specific pictures...
Good luck and let us know what you find!