I suggest you draw up a detailed plan and decide how you want the cabinet to be. After that then decide whether you want the chiller to be inside or outside.
And talking about using your air con as the chiller, you will be surprised actually that not all air-conditioned compressors are able to be utilise or split into aquarium and household usage. This information was told to me by a Daikin technician but I have forgotten which unit of Daikin's compressors is unable to do so; forgotten if it's Inverter or the normal one (: So word of advice is to check out your air con first before making the next step (:
Just to share; for myself, I am using a 1/3 horsepower chiller for my 3footer. Crazy as it sounds but it gives me some kind of assurance that if my lights are hot, at least my tank is surviving. And of course, a temperature controller is a must in case the chiller fails and all starts to fall apart because of that. But if nothing fails and with taking all your equipments into account, a good sized chiller is a worthwhile investment. In short, I 'kia si'! haha
Also, I think in the air-con industry, you can still find 1/2, 3/4 and perhaps 1/3 hp Japanese or Korean air-con compressors in the market for you to DIY into an aquarium chiller (info fed by the same Daikin technician). I guess you must know where to look. Industrial estates are great starting points. Similarly, getting such chillers from the market directly the price is definitely cheaper than a 1hp artica or from other sources who may be a middle man for the job. So I suggest bro that you take some time to travel around and look see look see. I know somewhere near Bukit Batok there's such a shop. If you stay in the west, you can let me know and I can tell you where the place. Offhand, I suddenly cannot remember the building's name :X (really getting old :S)
Finally, starting a new tank is great! exciting and fun but do take alot of time into the planning. It may not end up as something perfect, but hopefully it lives up to your expectation.
Happy Reefing,
Marc J.