I think it may be risky to do what you are hoping to do. It is a really nice thought, but to surprise him in such an endeavor is risky; it would be and has been for me. Maybe he doesn't want it really, all sorts of issues.
Do a safe bet gift
I agree completely. My wife would not buy me any such thing, because she knows that she does not know what I would want. I appreciate the fact that she does not buy me such things because I would not want her wasting money on something I wouldn't like.
The only exception to this would be if you had heard him say that he really wanted X, and then you got X for him. But if X is used, then you have to know about condition or it will still be very risky. So I would not recommend that you do this unless you knew absolutely that he wanted a particular thing, and that thing was new.
I also agree that it is a really nice thought on your part. I hope your future husband appreciates such things about you.
In my opinion, the very best gift is something that someone would not get for themselves because they don't know that they want it. To give an example of this, I gave my wife a small Victorinox pocketknife, like this in plain, non-transparent red:
http://www.victorinox.ch/index.cfm?site=victorinox.ch&page=176&lang=E
I gave this to her before we were married. When I gave it to her, she looked at me as though I were crazy (though she politely did not say any such thing). I suggested she carry it in her purse, and then she would have a small knife, scissors, file, etc., whenever she wanted it. She carried it in her purse, as I suggested. Within about a month, she thought it was one of the best gifts ever. She still has it. (Victorinox makes good quality knives that last a very long time if not abused.)
Now, I am not suggesting that you give him a knife, but if you know of something that he will like if only he tries it, that will be best. This, however, is not easy; most of the things I give my wife are things I already know she wants, so there is no risk involved. That is probably your best approach, unless you can think of something that he really wants but does not know that he wants, like my wife and the pocketknife.
