Welcome to the site.
I have a number of similar barrels and so far am using them to collect the waste water from my RODI system for recycling around the house. I do at some point intend to use one of them for pure RODI storage though. Currently I'm using a 20 or so gal rectangular rubbermaid thingy to store & mix new water. I suppose it depends a little bit on what was in them previously. Picked up two last year at a garden center and they had previously held 'glycerine', a by product of bio diesel production I think. The two I picked up this year have some real oily (vegetable/fish oil?) goo in them. They are food safe, but I think I'm going to need a pressure washer or even boiling water/steam to really get that residue out.
So, a pressure washer would be a good start, if that doesn't do it, steam or boiling water. The ones you're looking at appear better for access since the entire top comes off. The ones I have only have two small screw caps on the top for access.
You might want to check around your local garden centers and/or on site beer brewing businesses. Could get a better deal price wise on similar barrels. You should be able to see two of my barrels in the photos below. One is blue barrel from garden center, opaque one is from brew on premise business and used to be full of malt or adjunct.
