That's neat! What motor and gear box is used and where do you source them?
The tube will get warm as the presure increases. You will see that when you top off a gun with high presure requirements. When doing a typical airgun it's no big deal. I've been lazy as of late, not wanting to fight traffic and not filling my fill tank at JB. So I pump my fill tank the last three trips to the range with my 2 hand pumps. After about 100 pumps I let them rest and cool off. The pumps are surprisingly durable as long as they are lubed. I finally changed most of the orings in mine recently (about 18 months) when I started to notice it taking more pumps to fill than normal.
Since it is a hand pump it will collect moisture and a lot more than you think, use a good filter and water separator, but try to keep the size small as now you have to fill the volume of that chamber and it takes time. It helps the water escape first if you have the bleed screw at the lowest point.
If the motor is powerful enough and you want to really shorten fill times find the intake of the hand pump and plumb some air pressure into it, only put in enough pressure as not to blow the seals or stall the motor. I've used as high as 60 psi with great success.
Oh one other thing I'm not sure if you are aware of how powerful that motor is but it can be. In order to push that pump up to 300 bar it needs to be able to force well over 200 pounds down on each stroke. Watch your fingers, hands, and limbs! I use a big red stop button within easy to reach if something goes wrong.
Because the pump has more power than you can pump
DO NOT WALK AWAY FROM THE UNIT! It can easily get well over 6000 psi. A blow off disk is a good idea otherwise hopefully only a seal gets blown.
Oh, as far as getting a 3 stage versus a 4 stage. I would say get the three as there is less pumping. Get the 4 if you think your motor is not strong enough.
https://www.airgunnation.com/members/blackice/ has more long term experience when it comes to using a automated hand pump. I think he has had his for several years before I made my dual pump setup. I tend to abuse mine a little more. Especially since I used it to pump up my fill tank.
Allen