I had to pump to boost my supply and having already bought one would have been handy (cause it's also handy to just have in the house anyway even if you don't use EBM- I've pulled it out to get some milk for gunky eyes, express when a bit engorged). I wouldn't bother with many bottles or a steriliser though. As we just use it very infrequently I sterilise by boiling in a pot on the stove- no extra gadgets required.
Oh, and I wouldn't buy formula just in case. Having had very low supply and ending up needing some formula for a few days take it from me you get PLENTY of warning and working through it before you need to crack open formula if it comes to that.