I think 8 speed and stamina is a good place to start. However, my understanding is that most teams don't meet such prerequisites. I mean, it's expensive to build a perfect team like this, not to mention that sometimes the right players are not available on the market. So, most teams end up having a couple of players with 7 stamina or even less stamina to fulfill their needs. The chances are you aren't at a huge disadvantage fielding some 7 stamina players.
Well, if you have the money to improve your team's speed/stamina without harming the other attributes and you're willing to do it, why not? But it depends on a number of factors. Like, how hard is your division, how experienced are the players (some old players with bad speed/stamina will outperform young players with good speed/stamina), play intelligence, how much money you're willing to spend, perhaps you have supersubs for the players with bad stamina, and so on.
Regarding the other question, the difference between 7/8 stamina isn't huge. From what I understand, your players get drained as the match progresses, and what this means is that all attributes are adversely affected as the match progresses. More stamina would make those negative effects appear as late as possible. 8 stamina vs. 7 stamina would probably save you a couple of in-game minutes, but it's not a huge difference in my opinion.