Ultegra 6800 & 105 5800 compatibility

I'm doing an event in May which has some very steep gradients, and will be changing my cassette for an 11-32.

I believe this will need a longer chain and medium cage rear mech too. I have Ultegra 6800 on the bike - could I fit a 105 5800 rear mech (medium cage of course) to save a bit of £££ ?

The mech will only be needed for a couple of events a year I think so I'll be switching back to my short cage 6800 mech with the 12-25 or 11-28 cassette for the rest of the season...

Answers on a postcard please....