The instrument rating is tough, but it is also very manageable. Funnily enough if you get the basics right it will make everything so much easier. Practice having the aircraft straight and level and know the attitudes for climb/descent/ILS/1 engine etc and all should go according to plan.
The mental arithmetic is not too difficult, but try and jot a few things down on paper before you go flying. For example headings in the hold, timings on the inbound and outbound. They're not difficult to work out but with so many things going on you can easily get muddled up and I know one very competent guy who partialed on that very last point.
Another thing drilled into me was one thing at a time. I.e. don't go bugging headings in the turn!
If there is one thing that slightly troubles me is the RT. They will talk to you when you really can do without it and they seem to drone on and on, but I guess you just have to learn to deal with it.
I'm sure your FI experience will give you some kind of advantage with things like S&L and attitudes.
Good luck and enjoy.