No worries mate. We all have to live within our means. I'd think about the 3.1 at least as far as planning, even if I only bought the L/R initially, as I'd want the matching center generally. While not critical, I think it is slightly better to have a matching center, having gone both mismatched and matched with several systems. That said the usual compromised center style speaker (horizontal placement) has issues. Ideally you want all three positioned the same, but tv sets get in the way of that (but not acoustically transparent screens for projector setups).
I'd be more interested in your actual choices of components available to you for speakers subs and receivers. Much available on your used market? You can pick up some very good money savings that way, speakers tend to last a very long time if not abused, same for electronics (like poor heat management). OTOH you may just be happy with one of the HTIB kits if you're not as critical of audio as some of us