Yay! Skitarii!! (There's three i's in skitarii, by the way, not that it really matters

).
Man I love this army. And competitive? I've probably won about 97% of the games I've played with these guys.
Modelling wise, there is just a ludicrous amount of things you can do.
Two main paths here - go the 'regimented' look, which is more expensive in real world currency terms. As an example:
- Model carapace armour and bionics on everyone, maybe give 'em robes (take a look around the warhammer fantasy model range for ideas).
- Make your vehicles high tech - plasticard and space marine vehicle parts are the go! A predator turret on a chimera can look awesome, especially if you give it extra antennas and the like. Use space marine hunterkillers instead of the IG ones. Things like that.
The other option (the path I took) is more orky (conversion wise), with weird bionics coming out the wazoo. This is a cheaper option as
- Every model in my army has some kind of bionic conversion, but they are all different. Scrounge what you can - look at the necromunda pit fighters range, ironhands bionics, and so on. Really go to town if you want! By and large however, don't overconvert - they still have to look like a functioning, regimental army.
- My skitarii force was pretty much the first army I worked on when I got back into 40k, hence I took a more haphazard approach. My latter conversions are a bit more measured, controlled, but as a whole the army looks very cohesive with a standard colour scheme etc.
Either way, if you start to make a skitarii force, regardless of which codex you choose to use (I still believe the IG to be the best at the moment, you can always ally inquisitional 'counts as' forces if you need to), be prepared to scrounge!!!! Everything that is adeptus mechanicus is what you're after. Techpriest/marine poweraxes, powerunits for space marine dreadnoughts.
Actually, the adeptus mechanicus symbol on the back of the dreadnought is a perfect size for cadian plastic shoulderpads, if you know how to greenstuff mold. Not sure on the legality of that, however, so pretend I didn't write it.
My next project is to make a bunch of ratlings (as you mentioned before) and a basilisk. The ratlings are going to be similar to the Tau sniper drones, only much smaller, using servoskulls and space marine scout sniper rifles.
The basilisk requires me to wait for the new quad thunderfire thingy for marines to come out, i'm putting that gun on a chimera/basilisk chassy - I think it would make an awesome adeptus mechanicus basilisk. Or gryphonne mortar.
As for doctrines, well, you can really use whatever you want. Just because you have modelled bionics on everything doesn't mean you have to take the cyber enhancements doctrine. Sure, it IS fluffy, but for the reasons you are converced about, its not very competitive.
I Still use it from time to time, however.
Good doctrines to choose from include techpriests, iron discipline (maybe chem inhalers instead), actually, basically anything you think you could justify.
If you are making a fully equipped army, mechanised and carapace armour are often touted as being mechanicus (they get all the best stuff).
I even take drop troops, as it represents my units teleporting into the battle from orbit.
Basically anything can be justified. You wanna take ogryns? Make some uber battle servitors and there you go! You want grenadiers? I use these guys as inducted rouge trader troops. Some people even run their whole army as grenadiers, using this troop type to represent the average skitarii soldier.
As for worrying about a new codex, well, firstly its a fair way off, and secondly, I think an awesome skitarii army will remain awesome regardless of how the rules end up going.
Sorry about the unstructured form of my reply - at work, and quickly writing a sentence here and there when I get the time. If you want any particular part of the issue in more detail, just let me know! Plenty happy to help out. Especially if/when you decide what style of army you wish to make.
