Get a map, visit Hong Kong (give it a good 3 days), and arrange to meet up with people you know who have lived here awhile or local friends with expat tastes. Zoom in and decide on a district first (don't even say shortlist 2, just decide on 1), they go in and ask a local agent about the properties in the area in general, which would suit your budget and size and other requirements.
It will be too early to look at actual to let apartments until early September (no landlord would commit to letting you an apartment in October more than one month in advance), but what you can do is, if you see an agent you like and who is good, to show you some apartments on the market now for sale and to let, so you have an idea about the apartments in that building. He will probably do it if you tell him when you do come closer to the time again, you will go through him to rent.
Then when you actually come in October, you can just focus on the one/two buildings you like and see what is available, view those apartments, and negotiate and sign immediately.