I can understand why someone new here has a hard time making decisions as the scale of product value is completely skewed.
For instance, we often read how critical the amplifier is. Pushed to the extreme, we get the silly argument that you should pick an amp first and a transducer next. The truth is, a lot of this is often related inflatuation to justify expensive purchases else heavily influenced (i.e 2nd hand) opinion based on too much reading these forums.
We don't hear such claims about the source being critical as often and my best guess here is that it's because there are many more amp builders adversiting here (directly or through word of mouth) than high end source makers.
To answer the OP about the pairing of an imac line out with a lamdba rig. Considering you have made your mind about the type of sound you're looking for, I can only encourage you to pursue this.
Surely, I can imagine that the limiting factor will be the source but the good news is that it won't suck nevertheless (read about David1961's pairing of an ipod line out with a BHSE/009 rig). You'll have plenty of choices to explore new source later on.
If you start with an average transducer, no amp and no source in the world is going to transcend it. In terms of resolution, cleanliness, precision, there is no question estat is the way to go, whatever you read in the forums about the latest and greatest dynamic, isodynamic and other BA.
Also, my best advice is to take the active/hot product discussions here with a good pinch of salt as the posted impressions often are heavily tainted by new toy syndrome / self-justification / sheep mentality.
The stax threads are not immune either. We get so much propaganda about how critical it is to pair a 009 phone with a 5 grand aftermarket amp. Truth is, once you do the test yourself, you realise it may not be so, at least to your ears. For me, from direct experience, I am now more keen on spending 5 grands on the source than upgrading the amp. Thing is, there are many more stat amp vendors / posters related to the vendors here than people related to dac/source vendors...
So in the end, by all means, keep you own mind open, and try the gear by yourself. For all I know, you might hate the "thin" sound / lack of body from stat sound (the most common reason for people to prefer orthos to stats).