Hay, Hardy: I appreciate your desire to cut your costs, but an island location for your ventilation system is obviously going to cost more just due to it's nature. You can't get something for nothing. Now, that said, I would recommend that you dig deep into your investigations. You can do this at a good kithen design center. These centers ususally sell the best, and a range of the best. Don't skimp on your system. It takes some power to remove kitchen fumes and exhaust them numerous feet.
Just a few suggestions. Use a "remote" unit that mounts outside if noise is a consideration. Even then, there is noise........
Size your unit to 'do the job'.......don't expect a 500 cfm unit to handle a big job. Go for a respectable unit that is sized to "suck".
If your duct work goes thru spaces not heated, attic, etc., insulate to minimize condensation.
There are "down draft" units that "pop up" out of the top of your counter.....they work good, too, except I have a problem of trying to exhaust fumes that are trying to go "up" by trying to take them "down". But, if it is easier to go down with the duct, this is a very good option.
And,if cost is "really" an issue, then don't put your unit in an island. Redesign your kitchen to keep the cook top on the outside wall. Every time I see "perfect Martha's" kithen, I think why did I do it with an island cook top. Actualy, I do know................friends and family love to hang out by the cook top when you are prepairing food, and vanish when it is time to do dishes. Therefore, people put the cook top in the center so they can face their guests, rather than having to turn their backs if the cook top were on the back wall.
In the long run, the extra cost of having what you want outweighs a few bucks. You only live once..........