Engagement! Some bling for him is this a good idea?

Theresa
My boyfriend and I picked out an engagement ring together a few months ago. We had already talked about getting married and I knew it was in the cards for us, we already live together and we're very happy, but we've been together a little under 2 years. I wasn't sure when he would pop the question but a week after my own brother's wedding, he wanted to go ring shopping! We picked out a gorgeous two toned ring and he was a little unsure if he did the right thing by having me help pick it out and not just surprising me. But, I assured him I liked the idea so I could try rings on and pick out the right one.
Then, once we found "the one" he had some fun looking at matching wedding bands. He said something funny along the lines of wearing his band when he proposes. I thought he was kidding, but he really wasn't sure...I told him the bands aren't until the actual wedding. He seemed a little disappointed, like he would like to wear something to show off our engagement and commented on how men don't get an engagement ring (he wasn't being rude or anything). I even have a girl friend who proposed to her boyfriend and had already bought the ring for herself. Now, that's not me, but I appreciate all ways a proposal can go down!
So I had an idea...I was thinking I would buy him a nice watch. We've looked at watches together and I know specific things he wants in a watch, and that he wants a nice one. I can't afford a Rolex or anything crazy (I'm a grad student) but I've been looking online and I found one that's just what I would like to get for him, for a great deal on eBay. It even matches my ring, and when we picked it out he said he would want to eventually get a watch that matched our bands. 
So...is that a good idea, to buy this watch I found, and hang onto it until he proposes to me...then give it to him? So he can wear some new bling, too? His birthday is coming up, as well, but this watch is a little pricey for my budget and it seems fitting to do it this way :D