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I have a pile of extra chaos warrior, knight and marauder parts enough for several units worth of heads and weapons+arms so I'd only have to make a body, some armor and legs. In the past I've only sculpted Space Marines (fairly easy flat armor), fur, bases and gap filling. Have never done full bodies with proper proportions before.

Was considering making 2 units of marauders & darkoath chieftan with the marauder bits, and perhaps blightkings with the chaos knight parts. How much effort would it really be to sculpt all this? Anyone try making units of new models with putty and extra bits and found it worth or not worth it?

It totally depends on you and how valuable your time is.
On a $€£¥/hour basis it will not be efficient or cheap to do it well.
You'd be better off doing some overtime, selling some stuff including the spare bits on ebay at a car boot or whatever, not going out to that expensive restaurant or having that extra pint or two. Then just go and buy the kits from somewhere cheap.

If however you fancy the challenge or think you can do something special then go for it. I see little point in going through the effort of sculpting so many carbon copies of the GW miniatures.
The blight kings may be worth a go as they are low model count and nurgle sculpting can be quite forgiving.

I have done this several times, incorporating bits from various left overs into new units. Sculpting them takes time and patience. In the end it can be great to have a unique unit on the table, but just be aware of the time crunch. I have stuck to smaller units like command squads. Here is an example, and so you know, each miniature took about a week, with the actual sculpt time 2-3 20 minute sessions a day.
http://edwardmystcreations.weebly.com/imperial-guard-sisters-of-steel.html

I find legs and torso easier to sculpt, depending on your detail level. I also recommend you bit legs and repose, the work over them if you can for the first few. Working straight off a wire armature really takes practice and skill.
I guess my bottom line is GO FOR IT!! I totally encourage you, just be aware it can be hard and frustrating. Share a WiP going if you do start this.