Mid-late 1800's is pretty late period, but why not.
You'll need to look official, so don't just stop at kosode and hakama. Possibly add a yukata (lightweight under-kosode) and haori on top with your mon. If there are a bunch of you doing the shinsengumi route, definitely go for a uniformed look. Look sharp for the Emperor.
For headwear, go with either a bowlish type jingasa or a simple cotton eboshi. Some tekka for the forearms might be a nice touch.
You can go with a store bought hakama, as the back plate would be appropriate for the period.
Here are some patterns for kosode:
http://www.raito.com/clothing.htm
http://www.dementia.org/~djl/sca/japanese/patterns.html
and if you search around
http://www.sengokudaimyo.com or the "sca-jml" Yahoogroup, you should be able to find pdf files of clothing instructions.