You dont need the Star system to flush the fluid out of the system. You can do it without. All you need to do is to suck out the fluid to the minimum level in the resevoir (no lower!) refill with new fluid and then prepare to bleed manually at each corner in the recommended bleeding order. Pump through the fluid until you get clear brake fluid in the bottle, whilst remembering to keep up a level of brake fluid in the resevoir as you pump the fluid through. Do that for each corner and you have sucessfully bled your brakes.
The STAR system is only required for activation of the ABS unit by getting it to pressurise the system as you bleed it and to flush the fluid in the ABS unit. With this method you will flush the entire system as there is fluid held in the chambers of the ABS unit itself, but to be honest there isnt a great deal of fluid in the ABS unit and the typical flush doesnt really require it to be changed. Typically you would only use the STAR system to bleed the air out of the ABS unit if it is replaced or the fluid has been allowed to drain below the minimum level or if work has been undertaken on the upper half of the braking system, ie Master cylinder, brake pipes etc...
It isnt really necessary if you have replaced brake pipes after the ABS unit and the fluid level has remained above the minimum level. I use screw in plugs into the pipe ends that need replacing and so stopping the fluid loss as it is typically the rear rear brake linings that need to be replaced. If you do still have to get air into the ABS unit itself because of no other option then you can still bleed the ABS unit without the STAR system.
You can do that by replacing the parts of the brake system that are necessary and then once the system is resealed refill and do a normal bleed on the system. Then gently drive the van to a dusty dirt track or somewhere where the road isnt that great and cause the brake system to activate the ABS unit by locking the wheels on the loose gravel road or dust. Lock each wheel whilst doing that and then top up the brake fluid and gently drive back or bleed the system on location once again and you have sucessfully bled your ABS unit. Essentially you are replacing the STAR system to activate the ABS pump therefore pushing any air in the system out of the pump and into the rest of the system. You just need to be careful whilst driving because the pedal will feel spongy and braking will be poor until you have gotten rid of all of the air. If the pedal still seems spongy then repeat until you have a firm pedal.
Mercedes are crap and didnt design the pumps to auto bleed so that you had to take them back to a main dealer or buy a STAR unit. This is the most efficient way of doing it yourself and saving yourself a fortune in the process. It will also work if you cant get the system to work with the STAR unit properly. Incidentely if you do and can bleed using a STAR unit then ensure that you dont allow the ABS pump to run for more than 20 seconds at a time, they overheat quickly and they will fail.