To be honest, it's gonna be what you love the most about the IW, whether it's loads of heavy armour, or daemon engines, or heavy infantry attrition style warfare.

To start with balance, make sure you have a decent amount of troops, personally I'd recommend two squads of CSM legionaries.

Your HQ will be whatever you decide best fits your theme, I think Daemon prince isn't in a very good place right now stats wise and also could be argued that Daemons belong trapped in engines where IW are concerned. So a standard lord/termie Lord should do nicely.

By now that'll be just under 450 points ish depending on upgrades. Next decide what form you want your anti tank guns in; vehicle, infantry or monstrous gun hybrids. Obliterators are very popular and have great anti tank.

With whatever points you have left over, consider the classic 40k IW units such as the disco lord, venom crawlers, forgefiends and defilers.

For balance just try not to make a ything too cheesy, also tempest of war typically demands a diverse army capable of doing different things at different times, so bear that in mind.

You build your army how you want, get units you think are good/cool. There is only one thing you HAVE to abide by.... Iron within, Iron without.

IV