I'd say focus on lower body strength if you're doing it for a sport, especially rugby. In your case, you especially want a strong back, to keep yourself from getting hurt from getting tackled/falling down. In your case, the classic deadlift/squats should do you well. You're playing on your feet, so obviously having a more strong lower body will make you faster for short distances. For longer distance, then you really have to train the aerobic component more, but yeah. An interesting thing I read was (I don't have verification for it, so yeah) they put lower weight class Olympic lifters vs sprinters, and the Olympic lifters could actually keep up to 40-50 yards with the Olympic level sprinters.
As far as reps/sets/days a week lifting go, that's an endless debate, if someone found some super secret combo, everyone would use it, but it varies a lot on the person, what they're doing, etc. So while there's lots of recommendations (ie, 5x5 is probably most popular) you're gonna have to experiment and see what gives you the results you're looking for.
Generally, higher reps and lower weight builds more mass, lower reps and high weight, strength. Generally the lower reps tax the central nervous system most, in my case, doing that with mental health issues wassn't a stellar idea. By low reps, I'm talking only doing triples, doubles and singles for most things. It'll build strength (and mass), but the mental harshness of it can burn you out, so that's why people shy away from it, as they feel better doing high reps (hormones and chemicals like seratonin.) Westside Barbell, pretty much the strongest powerlifting gym around, does heavy days and light days, light days to increase speed. Westside methods might be something you wanna look into, they incorporate "the big 3" exercises, but they do have a successful track record training athletes with their methods, and they use a lot of assistance exercises.
As far as machines/isolation exercises go, they're not like...the devil, as lots of trainers (ala Crossfit/Mark Rippetoe/etc) make them out to be, but they're not really a good way to develop strength or very useable muscle mass. That said, I watched a video of Spain's national Olympic weightlifting team, and they have leg curl machines, should say something. Their purpose is basically, to use them for PT/injury prevention. Like for me, I'm using them now to rehab my knees. So they're good if you're using them in the right context, but bad if you're using them as the end all be all, which most people today do.