So I've started buying up stuff for my epic forces, and my orks currently consist of 4 boxes of E40k era orks and 9 gunwagons

By my reckoning I have something approximating the following: (currently in the post so I'm not sure exactly...)

24 stands of boyz
12 stands of grots
4 stands of nobz
4 stands of most other units (stormboyz etc)
4-8 stands of bikers (depending on how many per base)
4 stompas
9 gunwagons
Assorted Characters

my question is can this be organised into a reasonable army? and what else should I be adding to it? more battle/gunwagons? some SH stuff like super stompas or gunfortresses? a gargant or two? soopaguns?

I'd appreciate any advice you have on this!

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Well for one, you're going to need more nobz. Have you thought about stretching out their numbers with some regular boyz on their stands?

Stormboyz (and kommandos, though they're not used very often) come in mobs of 6, unfortunately. Bikers you'll probably want to stretch to 8 stands. For some of those you should PM Chaos & Evil; he may have some ideas about how to obtain nobz and other sorts of units.

If I were you I'd consider buying the following:
flakwagons
fighta-bombas (possibly optional if you get enough flakwagons)
dreadnoughts/killa kans if you want to use those stompas
buggies to go along with the bikers
maybe some transport

Gunfortresses and battlefortresses aren't terribly popular. Gargants may be, but that's up to you. It's a preference thing; using one will change how you use your army quite a bit. Soopaguns are important for the gunwagons but you're better off scratch-building them.

As above you need more Nobz to make a decent no of warbands as they come in a ratio of 2:6:2 Grot:Ork:Nobz. Though it is good practice to have a surplus of orks/grots as adding a few extra boyz stands(for the free grotz) to a warband greatly increases their staying power. The Ork character pack from GW does give enough bitz for 6 Nobz stands including a very nice warboss model, it is possible to pad these out to make loads more by using the odd boy or two.

Oddboyz are great, end of, you can't go wrong with a few of these wherever you put them. I use them in blitz brigades(gunwagons) and on big gunz stands attached to garrisoned warbands.

Fighta bommas are better value than thunderbolts and due to variable squadron size can be tooled to fly through flak, which is particuarly handy against the likes of Guard artillery and prepping for a landa assault. The +2 to initiative on the ground attack makes them a solid choice too.

I like great gargants but I usually tend to lean towards using an air waagh most games with Landas/FBs but they clash with the 1/3 aircraft/gargant allowance.

With regards to what you've got, a stompamob is good but works best supported by killa kans and a supa stompa. I've never used bikes. Stormboyz have scouts which can be handy, for all the usual reasons, but I find they're very fragile due to lack in nobz so best used against weedy stuff.

I could go on if you like but I feel like I'm rambling now.

What you have is a good start and genereally other people have covered most obvious things already, though I just wanted to comment that:

I've mostly seen Stompas run with a Super-Stompa rather than dreads/kans but either can work and they are usable at present, just very easy to break.
A Kult of Speed formation can consist of a mix Bikes, Buggies and Skorchas, but Bikes and Buggies are best run all of that type, so I wouldn't feel you need to pick up buggies to go with the bikes.

The Ork threads in the strategy and tactics section of the forum would be well worth a read through.