I'll try to help with this...
Many of the decisions you make regarding the purchase of a helo will be financial, and many will be technical.
Try this website...
Conklin and de Decker
I'm not saying these guys know everything, but they are a good place to look for "comparative" information on aquisition costs, and operating costs. Evaulate how you're going to pay for the helo, either with cash, leasing, or financing. Weigh the options, and consider ALL the terms of a contract VERY carfully.
These guys also strongly emphasize "purpose" when purchasing an aircraft, meaing what exactly are you going to use it for. This means checking out the performance specs of the helo, and its ability to operate in your area, and for the purposes you intend,(HOGE, HIGE, temps, range, VFR only, IFR capable, payload, etc.)
Decide where to hangar the aircraft, then check those terms and costs. Check the insurance costs on the various helos you are considering, because they vary. Check out both the quality and availability of the maintenance your aircraft will need, in the area where you'll be operating it. Talk with the reps at the service organization(s) you think you'll be using.
Check for quality and costs of conversion training. Talk to others who have operated the aircraft you're considering. They can shed light on any operational or flight issues with the helo.
Check the safety record for the helo. Particularly look for accidents that show "recurring" problems, as this may highlight certain weaknesses in the aircraft. Also look at the survivability rates vs fatal rates. Look at the overall accident and fatal rates per 100,000 flight hours and compare to other helos. Look at ease of flying for the pilot, especially if you're a low time pilot (insurance rates will be affected by this).
Check the airworthiness directives issued for the aircraft, and see if the aircraft you're looking at has all of the ADs addressed. If you find a specific aircraft you're insterested in, make sure the appropriate maintenance records are available, especially for the limited life parts.
That's all I can think of for the moment...
(edited for clarity)