In short, you CAN do it that way. But it might not be the best/least expensive approach.
A better bet for you if you insist on buying new is to get the 'home defense' version of the Express that comes with an 18.5" barrel and a factory Remington magazine extension (the 7-shot version). It might cost a bit more up front but it'll save you money and hassle in the long run, I promise. And if you want wood furniture, that's easy enough to swap out to replace the synthetic.
See the pic and specs at
http://www.budsgunshop.com/catalog/product_info.php/products_id/95077 , Remington has had a serious attack of stupids in their 2007 online product catalog and shows only a generic 870 pic for different models. If this is a sign of things to come with the new owner... well, let's hope not.
If you want to do the Lego thing on your own, your best bet is to find a good condition (inside, no need to worry about the outside) OLD Express on the used rack at your favorite FFL's establishment and buy that. Chances are it'll be cheaper in the long run too. See, if you buy the Express you're talking about, new, then you have to remove those pain-in-the-tuckus magazine tube dimples to put on a magazine extension. But the 7-shot factory version doesn't have dimples to start with- thus one more PITA hassle avoided.
The old Express guns were designed just like Wingmasters and Police guns, no magazine tube dimples and a spring-loaded detent in the barrel ring to retain the magazine cap. Newer Expresses have a plastic piece that is a combination magazine spring retainer and magazine cap retainer. Start unscrewing the cap on a newer Express and you hear clack clack clack, no such noise on an older one. Best bet is to pull the barrel and check for dimples, as long as the proprietor doesn't mind. Could be some enterprising soul already drilled them out.
You want the whole dimple story, go to the SEARCH THIS FORUM block and type in 'express dimples'- you'll get a boatload.
Anyway, you asked, there's an answer, FWIW. If you have any further questions don't hesitate to ask...
lpl/nc