The two cranksets use different bottom brackets, so you will need a replacement BB. Something like the SM-BBR-60 has an MSRP of only $32. It does need different tools to install than the old Octalink BBs, so if you are installing this yourself you will need to account for that.
The front chainring difference also presents a slight issue. The older Shimano groups had front derailleurs that weren't designed to shift a 16-tooth gap. Also, the R8000 cranks have the chainrings very slightly further apart than all the previous cranks (the gap between them is about 0.4mm bigger). The older FDs may not shift properly as they may not be able to move far enough inward (it does depend on your bike). So you would be better off getting a new front derailleur, although it can be a 105 version. If your current rear derailleur is the short cage version (which it probably is), then it may not be able to take up the chain slack with the smaller front ring. Also, your 9s chain may not work as well with the R8000 chainrings, but I think this may be less of a problem.
Last, if that 4iiii is a dual sided meter, don't bother buying it. Drive-side meters mounted on the R8000 and contemporary cranks are inherently inaccurate because the spider is asymmetric.
Overall, it would be simpler to get a pair of power meter pedals. Versions are available for Look Keo, Shimano SPD-SL, and Speedplay cleats. The cheapest of these tend to be the Look Keo-based ones, particularly Favero.