I'm Rimbo's teammate, I'll pitch in my two cents. mercadolibre.cl is your goto classified site, however most people in the program don't use it. There's also olx.cl. For housing homechile.cl is pretty good.
We arranged an apartment through homechile.cl before we got down. Honestly I would only recommend that if you've never really travelled before as the place we ended up getting isn't really the nicest. Most people come down and stay at someone else's place, their padrino's (a local arranged to be your goto person) place, or hotel/hostel it for a while. I'd recommend to take your time once you get here finding a place to stay as there are some amazing apartments that you may miss out on if you search online.
I am going to note that you should come down with at least 10k in your pocket. The first reimbursement is very slow, it usually takes about a month and a half to two months. This is because you first need to get your RUT card (identity card, you use it for everything), then have the program prepare the contract, then sign the contract, then wait for their lawyers to sign it. After that (about 3 weeks) you are officially in the program. For our round we could expense things up to this point however a round 3 wave 2 participant told me that they could NOT expense anything until this point as a rule change. This means all those expenses are out of pocket. The reimbursement phase takes about 2-3 weeks and you will NOT get all of your expenses approved the first time you submit. As an example, we only got about 52% approval. There's a lot of little things you need to do for every expense and it's really hard to keep track of them all.