With the exception of Felder, SCM and Hammer the following would be my personal views
1. Avoid any modern machine with lifting tables, I've looked at many and owned one and the conclusion I came to is that the structure of the machines is too light to hold the planning tables ridgid and allow repeatable lifting causing the tables to go out of alignment.
2. Remember to pay for the machine and not twiddly stuff. By this I mean anything that you are paying for accurate thickness settings is not worth it. For a project you will normally mill all of your stuff in one go. Since you are making the item as long as all the stuff is consistent it doesn't really matter to that its not 0.1mm accurate to the size specified. You can get to within 0.5mm using a micrometer if needed, but in reality, leave the stuff overnight and it will have changed its thickness.
3. The longest heaviest cast iron beds should be your priority. 12" wide is great, but most people don't use much more than 10" wide. The length of the beds significantly improves getting stuff straight. The longer the better.
4. If at all possible get a machine with a Tersa block. Simple and unbelievable easy to change the knives with absolutely no setting required. Spiral blocks are seen as a step up as they are quieter. However, for simplicity and ease of use you can't beat the Tersa system (IMO)
5. If at all possible get one with planer tables that will lift off, or better still pull back out of the way. This makes cleaning the rollers and cutter bock much easier.
6. Rigidity is the key, choose a machine where there are cast iron pieces bolted together that house the planer tables and thicknesses tables in one rigid solid lump. Yep they are more difficult to move around a workshop (you shouldn't really move them as they settle when sat) but you will spend more time doing wood work than fiddling around with a machine that produces taper cuts and needs all of the beds resetting frequently.
Recommendations,
Well, anything from Wadkin, Cooksley, Robinson, Dominion are all sound investments that you won't ever loose money in. They are however all out of business and they are only available secondhand. Plenty of choose on auction sites. For a modern machine, you can't buy a better machine at a moderate price that a British build Sedgwick. (I choose the 16" Sedgwick with Tersa and it's simply brilliant again IMO). They do a 10' 12 and a 16" version. Again also available secondhand, and parts are available should you ever need any (almost unheard of, they are built to last many generations of use). They are based in Leeds.
Top end stuff is Felder, SCM and the cheaper brand from Felder, Hammer. I don't have any experience of any of them, but they are primary foundation machines for serious modern industrial applications.
Extraction is best run to a take off point directly above the machines with a flexible hose down. This allows the change of side of the extraction bucket to be accommodated without any real problems. Mine is at head plus an arm height with a blast gate I can reach from the normal operating position.