I want to buy MIDI-keyboard to drive Soft Synth SI-Bass.
Does SI-Bass understand Velocity from MIDI-keyboard?
 
What features of MIDI-keyboard are vital?
 

Velocity is recognized. Pretty much any softsynth does that much. Other features I personally find important is a pitch/modulation controller, whether with two separate wheels or with a lever or ribbons is up to your preference. And having a few assignable knobs or sliders is nice when you're working with softsynths. You could assign these to various knobs and sliders you see on the screen and then you can make them part of the performance, or at least you won't have to mouse around all the time. For basslines, I'd go for 37 keys minimum, for full synth 61.

Thank you for help!
 
M-Audio Keystation 61es is good or to spend some more money and buy a better one?

I don't have personal experience with them so I couldn't comment on the quality, sorry.

I have an M-Audio MK-4-49C  MIDI keyboard controller that has worked perfectly for about 10 years now.  Based on this I can recommend an M-Audio MIDI keyboard controller.  

I suggest that you spend a little more and get this M-Audio 61 keyboard:
http://www.zzounds.com/item--MDOOXYGEN61IV

Sliders, knobs and in this case drum pads also can add a lot when recording MIDI tracks. I do believe that if you don't get a MIDI controller with assignable  sliders and knobs you will replace it very soon in IMHO.

If you have such possibility, find some place to test different controllers. They are quite different in felling, behavior of keys and even the size of keys (all "full size" keys have close to the same width, but not the length...). Everyone has own preferences there. In case you get "wrong" one (for you), controlling velocity can be difficult.
 

How is about Roland A-49?
 

How is about Roland A-49?
 

From functionality of keys, they are all the same (till Aftertouch support). But the keys are (mechanically) different. Roland MIDI keys are "short" (on all controllers I have seen) but relatively "heavy" (for "light" keys, hard to explain). M-Audio has Oxygen with "light" keys and Axiom with "semi-weighted". Akai has good "semi-weighted" keys. In case you try them, you immediately recognize what I mean, it is not like compare hammer actions of DPs in the same price class, since they all should imitate real grand piano. For MIDI keyboards there is no single "reference" and producers do whatever they like.
 
Pitch/modulation controller implementations are also different.
 
If you want something with controllers, you should take SONAR compatibility into account. While you can configure any, CW keyboards, Nektar (Impact only at the moment!) and Novation (with AutoMap) are "SONAR aware".

I bought the Roland A 49 last winter after trying a bunch in a store because it had the best "feel" of all the brands. I found most to feel like toys, cheap. They had all major brands. Korg was also good but there was no controllers there. Roland has made keyboards longer than all these other brands and even though they are now made in China instead of Italy, they  still feel the same quality under your fingers.  
 
What aszslow3 said is excellent advice. A keyboard is an instrument like a guitar not just a bunch of buttons you push, feel is super important if you want to improve your chops. 
 
Also consider your studio set up before you grab a big keyboard. Mine is the perfect size to sit on my right side table next to my Screens and computer keyboard. 49 keys is more than I ever need for my piano, organ and bass parts. 
 Read my review here to find out the details of the features and why I'm a happy camper: 
https://www.gearslutz.com/board/reviews/917212-roland-49-a.html

 
Best of all the Roland A49 was only $139 and came with Sonar X1LE and I upgraded to X3 studio for $99. So the keyboard actually cost me nothing if you consider what I saved on Sonar.