ooh, cool plugin! I've been thinking Guitar Rig is getting a little long in the tooth. I'll give that a spin tonight to see what CPU use is like.
If it's light for a desktop, it should be alright, although obviously it won't be as fast as a quad core Xeon. Speed should be roughly comparable to Ableton on a similar machine. Regarding the types of plugins, as I mentioned I don't know Helix Native, but Guitar Rig is comparable product and 3-4 instances would definitely be feasible. Ditto Kontakt, depending on plugin involved (quality varies here unfortunately, I've seen some plugins with very low usage, others that do a lot of DSP [badly] can be quite slow). If you're using fast algorithmic reverbs (I personally love Valhalla) you should be alright.
One other thing: ALK makes it very easy to route things in a sane way. This not only makes your project more easy to navigate and logical, but it lightens the load on your laptop also. For example, usually one or two verbs suffices; one is the "space", the other more for effect. The first can often simply be a send, so one verb is shared across all tracks. You can also share guitar plugin instances when you're looping: just route several raw audio tracks into a single, "plugin" track, mixing them all together in the plugin track.
Finally, since you're doing loop based music, you can always loop the post-processed audio; in that scenario you have one or two guitar plugin tracks (round-robin if necessary), which output to a waterfall of audio-only (i.e. no FX) tracks which loop the effected material. That way, you only use CPU-resources on the new material. This is obviously more work, so only do it if you run into trouble, but the point is there are plenty of options to reduce load, and we've worked hard to make those relatively elegant.
By the way, we'll do more work on that front sometime soon, like option to render looped audio, meaning FX are disabled during play regions, this would have a huge impact on performance. I think the scenario of live looping is actually quite good: the repetition can be leveraged to reduce CPU load. So I'm sanguine about optimization possibilities.
With regard to the model: the higher the clock speed, the better. E.g. there's a 3.5Ghz MacBook Pro (the top-of-the-line one). Memory is less important by the way.
ALK currently has a single-threaded audio engine for now, although the app as a whole is obviously multi-threaded.
