I wanted to compile some general advice in building a new Ork army, and things to think about to keep it affordable. Feel free to add in the comments.
First off orks live off kitbashing, many sets can be turned into multiple units with some planning beforehand (I know planning.. cchhhrrr-PFF, but let Mork take over for a bit.)
Kitbashing needs two components, Bitz and Bodies to stick them to. Orks mainly have Boyz bodies and Nobz bodies. The new wave of miniatures have made them somewhat larger, but it's barely noticeable.
The older kits are often cheaper and with more bits to use than the newer ones but some look dated. My advice if you want the newer look but still keep your teeth: get one old box and one new, stretch the important bitz on all the bodies (mostly heads and weapons and more dynamic poses)
Here are some Kits I think are worth it for any Warboss just starting out to get loads of shiny bitz left over to use to maximise your playable units.
-->Old Nobz -they have static poses but they give you 5 Nob bodies and so many weapon bitz it will fill your Battle-Moon. Also Armour plates for days as well as trophy poles that can work as more armour, and decoration to "orkify" looted vehicles and the like. Combi weapons serve as access to flamer and rokkit Bitz, and Choppas and Power Clawz are great all around (two one handed choppas heads make for one interesting and intimidating 'Uge Choppa for example). You get five of each weapon option which is immense. Also many decorative pieces like Pouches, ammo and trophies. They also easily stretch one unit of (currently expensive) flash gitz into 10 guys, or convert it from scratch, just Dakka-up their Kombi weapons.
-->Deff Dread -its basically the Vehicle equivalent to Nobz. Loads of ranged weapons, loads of Armour plates and orky decor. (Use it to make vehicles from other armies into Trukks for example -Taurox, Chimaera, Rhinos etc.). Many new vehicles have more weapons than old ones (or looted ones), this kit gives you the rokkitz you were missing or the Buzzsaw you never had.
-->Boyz - get yourself some Boyz. Be it a cheap old Boyz box, the Armageddon Boyz or Boyz from the excellent old Combat Patrol. These are loads of cheap bodies that work extremely well together with other kits (Wrekka Crew as is next). The Nob or Loota kit gives you the Shootas/flamers and rolkits that the older kits are missing. Got some rolkits? - You've got Tankbustas, some trophies and fancy choppas?- Beast Snaggas
-->Wrekka Crew -yes it's an expensive Kill team Boxset but it hast the Bitz and the looks. Together with some Boyz and some proppa finkin. You get yourself a unit of Breaka Boyz and easily two units of Tankbustas which is hella points per dollar. (Even more easily done with leftover rokkits and armour from the Nobz)
-->Mega Nobz - again Amazing weapon, armor and Back Banner bitz. But it contains most importantly very special Big Mek Bitz that you get nowhere else. You get a choice of Kustom Force Field (to kitbash a Dakkarig or some other orky science Project of your own) or Tellyport Blasta (as extra fancy dakka, also makes a good Shock attack gun on another Nob Body). +Welding Mask, Hydraulic claw/drill ...Use one of them on your Big Mek and the other somewhere else, or both of you want to build 3 MANZ.
honourable mentions
--> Lootas - they are with Gork and Mork now, but that can make them extremely cheap on the secondary market. They have amazing bitz to make anything Mek related, interesting Dakka Bitz for all your needs as well as Boyz bodies. The spanna gets a choice of pretty much every heavy Ork ranged weapon and you get Flamers en masse and Orkey looking barrels and pipes for vehicles.
-->Mek Gunz -they are terrible points per money wise, but contain a load if fun weapon options. With some wheels from some toy car and some sprues you can easily make 3 out of one kit (even easier with deff dread bitz) (no one knows the different guns, just play them as you like). Or you can use the guns for your own creations.
->Also they have enough gretchin to make a whole new unit if you have a few more gretchin tokens or saddlegitz or something which is nice. You don't need them for the Guns to be playable they are just tokens.
-Flashgitz of course have loads of awesome Bits that are useful for anything mek or dakka related.
-Armageddon Charakters (on the secondary market)
They are cheap and give you good bitz and most importantly Bodies. Big boss, Painboy and Banner Nob are amazing and unique Nob Bodies as Tankbusta/Breaka Bosses or as additional nobz for your Boyz.
If you are at a point where you don't need all your units to play, just mix and match models to create more unit diversity.
-Boyz units have 1 Rokkit per 10, 5 of those(or spannas/lootas/any boy with some heavy ranged) +nob (with Kombi rokkit/Bigchoppa as Smashhammer) make a unit of Tankbustas... and the other way around.
-Also Boss Nobz (from Boyz/Wrekka Crew/Wartrakk/Flashgitz/Big boss/Painboy/Banna Nob) make a whole Nob unit pretty fast
-Gretchin tokens/decor on other units make a whole unit at some point as I have said.
And I'm sure there is more. Planning these multi use Models makes this a lot easier, magnets can help here as well.
For kitbashing just stick with the relative size and similar armament (Big stick means good in melee/big gun means Dakka, no one likes to get Krumped up close by a guy with nothing but a gun) to the real thing and you should be fine (be upfront about your Multi Uses and Kitbashes).
Be smart, use what you have, and remember: finkin' and taktiks may not be proppa, but you don't only have your 'ead for butz.