Felix,
I completed both my CPl and MEIR part time due mainly to work commitments and am living proof that it can be done in minimum hours on both.
As your question highlights the IR I will limit myself to that.
Can it be done part time? Yes
Can it be done part time in minimum hours? Yes
BUT
I cannot over emphasize the following.
I worked to a point where I was making myself ill. I was working a full time shift work job and for the 6 months it took to complete the course I estimate I was working a further 20-25 hours per week on top of my day job.
Instrument flying is a perishable skill and learning instrument flying is at least 10 times more perishable.
It doesn't matter how much you read your books, every day you sit on the ground your scan, knowledge, RT, spacial awareness, hand eye co-ordination and spare capacity will start to fade. Most full time students may go a day or so without flying, you may end up going 4,5 or even 6 days without getting a sim ride or airbourne.
The fact that days at work get in the way means that you can end up with very long flying days with 2 or even 3 sim trips in one day.
You can also end up in a position where you get few backseat rides. The back seat ride is a highly understated learning tool and believe me every little helps.
A guy who did his IR alongside me ended up struggling due to lack of consistancy and changes of instructor. 6 months in flying training is long enough to see staff changes as instructors move on! Extra hours in a twin are very very expensive. A fail or partial at the IR could easily cost £1000 because you may not be free to have another go tomorrow, thus losing currency at a crucial time and therefore needing an extra pre re-test trip just so you are up to speed going into a test.
An average full time IR takes roughly 6 weeks, part time I would look at closer to 5 or 6 months.
Budget wise I would add at least 10% flying hours and money from what I saw of my colleagues and possibly as high as 20% extra.
As I say, yes it can be done, and in minimum hours, but please consider the challenges you add to an already demanding course. Look at how easy or difficult you found previous training, consider what you will do if you aren't ready for test come the 50 hours of training, or if you have the capacity financially if you go over budget.
Regards,
VFR