I had actually looked up what you are thinking of just shy of a year ago Stormwind. Collecting all of the pieces always came out to be more expensive then a completed set. I think 80 bucks was where I was at with the price on ebay. Show a little patience and one might pop up, do some research and find a list for of parts for yourself to look at to do a checklist on the ebay pictures to see for yourself how incomplete they are.
If you do end up piecing it all together you will either need to find a LOT of mini's to replace with something else or buy the actual things piece by piece like you say, very expensive.
The deeper you dig you will also find that most of the die hard players have actually replaced most everything with something else anyways so if you feel you would be willing to do that you could. I already have plans to have a 3' by 3' board printed off so I can cut out the rooms and make a modular board on paper steel and then magnets under all of the mini's and a magnet sheet to lay out first, underneath it all. "Plans"

In the end I think you are looking at storage, if you want the original you can get a box with some terrible storage or if you go full custom you can make a new box with everything in it.
In my experience, I like to start with the original and I waited almost a month and a full set showed up with 1 picture of the box and one picture of most everything hidden underneath the dungeon master screen. The seller had no idea what it was and even though a few blind bids were put on it, I took a leap of faith when no one else did and got a completed set (minus 1 magical item card) for not even 50 bucks shipped. I even received in the mess of stuff, the mini's from what I have learned is a second edition D&D promotional box of pewter miniatures. That being said, the board is warped and the box is barely together at all anymore and I have 2 broken orcs.
Again, 80 bucks, 100 pounds? I think would be a fair price for a completed set on ebay but my maximum, about.
Also, a fan note. The American version is the hardest as the treasure deck is a 60/40 of good to bad. I believe there is a "European" and a "French" version that is at 70/30, good to bad events. Something with the monsters as well.
Check out the Heroquest forums here, me and Irisado talked about it way back when I was in your shoes.
PM me if you want a set of the original rulebook and quest book, I have a pdf you just print out and fold in half and staple and you have the exact original stuff. oh and I found a link to a place with everything you can print yourself if you wanted to build your own expansions to the original.
http://www.heroscribe.org/cycoregamecrypt/maps4.htm