The cosworth 2.3-16 makes a fair bit more power than the 2.6 (190hp vs 160hp) and is the peformance version, the 2.6 is the luxury version so both of those are the best two of the standard deliveries, with the most delivery options and best performance. There are also the late build collectables Evo I and II versions in 2.5-16 but they're expensive, rare and in demand.
The W201 in 180E and 190E versions did come with a number of engines ranging from 1.8 litre to the 2.6, most common for the shopping trolley 190E is the 2.3 SOHC M102 motor (113-130hp depending on year). Personally I'd avoid anything smaller than the 2.3 so check a used example isn't the 2.0 or 1.8 litre (also used in the lower-spec 180E, same car but base version with few options or features like power windows). The 2.3-16 and 2.6 are easy to tell at a glance because they have a trunklid badge, just check on the engine size if there's no badge, I'd hate to have this car in a 2.0 litre it'd be a slug.
AMG released a version called the AMG 3.2 which is based on an M103 six in common with the 2.6 litre, and a very popular home tuners modification is fitment of the 3.0 litre M103 used in the 300E and 300SE W124 sedans, which basically fattens the torque curve and gives 20hp more, and it's a bolt in change using the same anciliaries, the only real difference between the 2.6 and 3.0 is bore size and intake valve diameter, and the spark controller has a different timing curve (it's also a bolt in change). So revheads like the 2.3-16 and 2.6 fairly equally too.
As far as engines go the SOHC 2.3 feels like a four, the 2.6 feels like a small six and the 2.3-16 is surprisingly quick. The SOHC engines are tuned for best performance at high rpm and economical performance at low rpm so they're a bit split-personality, the 2.6 does have enough flexibility and power to feel quite quick especially with the very low drag coefficient the W201 design has. The little six has just enough guts in it to find some conservative race speeds and really be impressed by the W201 design, it only needs about 140hp to crack 200km/h and most contemporary body types need more like 200hp to force through the windspeed, the design intention to race these is clear when you push them.
You'll never really quite see this side of a 190E with the SOHC 2.3 so that's my biggest argument for the 2.6 or the 2.3-16 if you can afford it/find one. You can find the 2.6 cheap and there's plenty, and it's a terrific engine. For an experienced driver you can make it act like any contemporary 3.0 litre V6 most of the time, the 2.6 punches well outside its weight class and returns 30mpg. Up long steep hills or at low rpm it does feel underpowered however, there's a bit of driving technique involved but it responds and delivers when you get to know it.
Things to watch out for are a leaky fuel distributor (expensive and tricky), transmission problems, any trannie problem (ridiculously expensive to replace or recondition), fuel pump leaks (they leak for a long time before they let go, must be checked physically), and timing chain seal at the front of the engine tends to go. By reputation they go through head gaskets sometimes, so check the plugs to make sure they all look alike so the cylinders are sealing okay or at least wearing evenly and it's not overheating. Take for a decent drive, on a warm day it shouldn't go much past 80C at idle and at speed it should level back to just under 80.
The rocker cover gasket tends to go with age too, it's rubber so it petrifies and leaks then makes the whole engine look leaky. But that's a cheap fix and other things I just mentioned are expensive to fix. It's a good excuse the repaint the rocker cover while it's off, I did mine in a bright red that looks superb.
Other than that there's just regular buyer concerns like rust and general wear (suspension, brakes, steering, etc.), common to any used purchase. They're not particularly more expensive to replace in this model than any other make, I got new brakes for $300, new steering same again, etc. It's not like what general repairs on a current model will cost you.
Another buyer concern is specific to the make/model is the very wide variety of specifications and options in the model. It just means you have to use your eyes and don't necessarily believe what the seller tells you, someone might try to pass off an unbadged 180E 1.8 as a 190E 2.3 but you can pick it straight off by the wind up windows. Some export deliveries like those to asia don't have leather, so they're not worth as much.
Do a little research on the options and specifications for the 190E on the market to give yourself an accurate idea of what the resale value of a given example is actually worth to you before handing cash over.
Mileage is a broad consideration, on one hand these great little cars can handle half a million kms and never let go, they're bulletproof when looked after. Other times they can be fragile at much lower kms because the owner is a ****, so assess the specific car for both mileage and the maintenance condition the owner has kept it in all that time, or his treatment of it.