The combat system of ME is way more action focused. Basically it works like a third person shooter. In ME1, your stats still affect your accuracy and damage a lot but from ME2 onward, it's basically straight up shooter gameplay, although you can pause the action any time to give orders to your companions (see below) or just to get your bearings.. In addition to shooting, you have special powers, depending on your class, which range from special damage types (fire/electrical, etc.) to crowd control such as suspending enemies in the air and similar things.
Also you cannot select your companions. Instead you give orders, where to take cover or which powers to use. But you will never leave the perspective of your main character, so you need to position yourself in a way as to have a good overview of the battlefield.
Dialogues are there aplenty and they are using the wheel, similar to DA2/I (in fact, the later DA titles brought the wheel interface into the series from ME). You'll make decisions throughout all three games which can carry over through the trilogy as you import saves into the sequel games.
In terms of story: I'd say expect a very strong start with beautiful world building and one of the most enticing scifi settings ever created, deep lore, beautiful visual style and fascinating new worlds and alien races. Expect deep and developing characters and thoughtful and emotional stories. But also expect a fair amount of inconsistency between the three games and expect the story to turn more and more from good old nerdy scifi to sometimes brainless action schlock as the trilogy goes on.
All in all, if you are asking whether or not you should give it a try, I'd say if you like DA and BW games in general, it's a no-brainer, just get the trilogy (and the story DLCs, which are all fairly important or really entertaining and add a lot to the games).
As for how best to get it, unfortunately, AFAIK, there is no really great way to get everything. There is no super pack that includes everything. Whatever you do, you will probably have to bite EA's bullet buy at least some of the DLC extra.
Whatever you do, I definitely recommend getting the trilogy for PC. The requirements are not that tough these days and you'll have the option to use mods, which can substantially augment the experience.
For example, for ME1, I do recommend using MEUTIM, the ME1 Updated and Improved Textures Mod. Don't even start without it. There are also lot's of upgrades for textures and even some content stuff for ME2 and 3 on the Nexus. For a first playthrough I would recommend to stick to the textures but it's worth checking the sections for the games on Nexus, you won't regret it.