Go for the IMC. It will improve the accuracy of your flying considerably and it will add to the safety of flying if you get caught out by the weather. Training is 15 hours minimum. The rating lasts for 25 month, when you have to revalidate it with an flight test.
There is no renewal or revalidation of a Night Qualifiacation. However, you have to have done one take off and landing at night within the 90 days before taking a passenger.
With a multi the currency requirements are 10 route sectors in the 12 months (route sector is take off, cruise of at least 15 minutes and landing). You need a proficiency check within three months of the expiry of the rating; alternatively you can fly one route sector with an examiner during the proficiency check in lieu of the 10 sectors. The multi is I believe type specific, so there will be additional cost in moving from one type to another