Here's a post about how Bing views keywords:

The Meta Keywords Tag Lives At Bing & Why Only Spammers Should Use It

So, over-stuffing your keywords isn't totally ignored and can cause a penalty on Bing.

Meta keywords were so abused in the past that they could no longer be used to accurately tell what the page was about. As a relevance/ranking signal it became worthless.

If you buy the domain and re-direct then remember to think of your competitors customers - not just the potential ranking boost!

Ideally you want their customers to become your customers, so think about how you might redirect specific pages from their domain to landing pages designed to reassure people looking for you competitors products/services and position yourself as a viable alternative.

The last thing you want if for them to think they're in the wrong place and bounce straight away, never to return!

Before buying the domain, can you get any supporting stats / analytics? How many direct visitors, how many via search etc.You're likely to have a small window of opportunity, but it's definitely worth thinking about how you can get some of those customers that you might not be able to replace just from search.