What kind of plastic are you using? if you use standard HDPE it is easy to mill you can almost use any kind of spindle or mill. If you are using different materials like maybe a PET(G) or a Polycarbonate i would recommend using a electric mill just because you have a bit more control over the Rpm's and could be very useful when you get unwanted vibrations.
required payload is a little bit harder, it depends on the kind of mill you want to use and the force that is needed to push the mill through the material. Maybe you can explain a bit more what you want to do.
Path accuracy of 1mm is difficult, and may not be achievable. Path repeatability is much more precise, but requires iterative program adjustment until the desired tolerances are achieved.
For this application, you probably want an HA-series KUKAbot, probably with a field Absolute Accuracy calibration performed using your working payload.
However, while robots have excellent repeatability, they lack the rigidity to maintain accuracy against outside forces. So your milling process would need to emphasize minimal force on the mill -- high RPMs, lower feedrates, possibly additional flutes on the cutter to reduce chipload.
The task would be something like in this video --> https://www.youtube.com/watch?v=VjAAhsrHwR0 What may be the robot model in that case?
Sorry... I ment robot model in this video. Who recognizes?
Thanks for the replies. In this case about +/- 1 mm path repeatability will good enough. It can be reached with standard model KUKA I suppose.
It seems to be a low payload robot like KR 10 or KR 16.
But also look for Robot specifications according to your requirement. Go through the technical documentation available on KUKA website for each type of robot model. You need to consider Robot payload, Robot Reach, Controller Type, etc for your requirements.
Thanks
It seems to be a low payload robot like KR 10 or KR 16.
Thanks 
At this moment I'm considering KUKA 16-2 (KR C4) and 500W electric spindle having speed range 5000-50000 rpm. Weight of the tool is 3,5 kg. Reach need to checked with this model.
go for KR30L16 and reach is noooo problem...
However, while robots have excellent repeatability, they lack the rigidity to maintain accuracy against outside forces. So your milling process would need to emphasize minimal force on the mill -- high RPMs, lower feedrates, possibly additional flutes on the cutter to reduce chipload.
The lack of rigidity can be compensated for by using sufficient amounts of complicated math 
One of my friends from the university did some research on that while he was taking his Phd. As far as I remember, he ended up milling in hard materials (steel?) with a KR120. As far as I remember, the tolerances were within 1/10 of a mm. They made their own calibration of the robot using some advanced optical rig. Impressive stuff.
another option if rigidity under load is a problem is just go bigger. We are using a KR150 (which we bought used and had been running 10 years)with a 5.5Kw spindle to mill through LDPE with speeds up to 0.5 m/s and mills ranging from 3 to 35 mm. I have had some problems with accuracy at high speeds in tight corners, but never with rigidity of the robot. I have done some experiments with aluminum and wood and there i had to slow down significantly but still no problems. General accuracy i have is around +-0.5 mm.
In your case this is probably a bit overkill to set it up this big, but i won't go to small. You talked about blow moulded products so is suspect you want to keep up with a blow moulding machine where cycle times range from 6 seconds to a minute. where a minute would be a big product. It would be unfortunate if you could not mill fast enough and can't keep up with production.
For the mill you should probably look at woodworking or aluminum mills. If you use those at high RPM's you have low pressure on your product and spindle and get a nice cut.
The lack of rigidity can be compensated for by using sufficient amounts of complicated math
Hai. Sorry for distributing your festival holiday. I doing robot milling using KR120 HA. I interested to know your friend Phd and possible to link me to him. My email addie@innopeak.com
I think Leon meant 0.05 m/s (3000mm/min) over soft plastic like materials should be easy. But there is always a learning curve.
The material we are talking here is quite soft, no external forces have any significance, and error on position or tolerances of + - 1mm are quite achievable, provided good practices are exercised.
It is and has been quite a learning curve for me to learn how important all the details are... just like in any machining.
I always remember (and keep in mind) many decades ago, that the one machinist in my home town that had the oldest and more basic lathe in town, whenever you wanted an accurate part within tight tolerances you will take it to him. You can recognize a message there.
go for KR30L16 and reach is noooo problem...
I would advise that long arm models are not desirable for milling projects
