You have one of the most unreliable Siemens spindle drives ever built.Even dropping a phase on the early ones destroys the ER unit.
There are often new in the box boards for these units on ebay.co.uk,they are a lot cheaper now than when I first needed them.
The problem with replacing it is it will be hard to match a competitors drive with the existing motor.
Cheapest option is probably repair.
My experience of Siemens is that they will give you options and specifics but they will be Siemens ones and you can`t expect much else.