The 'good' is   they  did quite well  when operated properly.      They can sometimes be  bought cheap, just because of  some  'bad'  rumors.

Nine-oh-nothing.  for example.

HP is just a function of  torque and rpm... so  when  you  throttle back and gear up  to  'save' fuel,     they tend to  lug down a little faster in tough spots.    V-8's are totally different animals  than  straight 6s.   

I've  got  2 903's,  and   several   perky  540's.      It's not  difficult to 'learn'  the differences,  and   operate  accordingly.  

They run about  300 less ft. lbs  of torque than say  a lower rpm'd  855.   But  when they are  'matched'  to the proper  drive ratio,    your 4840, my 4880,  for example,   the lower torque is  not really missed.

The 'bad'.. is   Cummins support.   ZERO.     It is  deemed  'obsolete'.   And,  not every  shop is still equipped to  do  PT pump service.

Aftermarket  is  even dwindling,    Interstate-Mcbee   pretty much  is  all that is left.       Master kits,  gaskets,   water pump kits,   oil cooler orings.     'still available'. 

Injectors,   injector pump service,   turbo replacement;   still good.

I just rebuilt   2 of mine last summer/fall.   Interstate McBee out of frame kit,    water pump kit..  had the PT pump  freshened up,   rebuilt injectors..  and new, not rebuilt turbo for.       $2350 for the master kit,    $125 for the waterpump kit,   50 bucks for  new thermostats,    My heads were already done from a previous freshening,  but  that cost me about 900  for  new valves, milling, and new cup install.    OH,  valves are still aftermarket.

Hard parts such as  gears,  oil pumps, crankshafts,  cam,  con rods..  exhaust manifolds..   hit and miss..  some vigorous  searching may find  some of this  stuff  packed away in some warehouse somewhere.     It's not going to be cheap.   

My  advice, is to snag a 'spare'  engine,  just to have on hand.    Last  one I got  came from a burnt  4880,   and  got it for about 850.  

I'm going to take a wild swing on the durability of  aftermarket parts,  but   I'm  hoping I can  get 5000 hours on a rebuild.     

All in all,  it's  a  simple,  dumb,  totally mechanical engine.   With a little  research,  anyone can  work on them,  and  run the overhead just as well as  any 'newbie' technician. (believe me,  most older guys  who cut their teeth on these engines are either dead, or permanently fishing)

For NOW,  I have no problem  relying on my 903's as my principal  horse.

Edit,  I'd be more concerned on  parts availability on the  TRACTOR..   before finding  parts for the  903.   Agco also has  turned their back on the  4000 series  tractors.   ( most recent for me is  the fact all the gears in the  axles are no longer available.)    ring and pinion,  final drives, stub shafts....NLA.

That's the  benefit of  having one  in a  MF 4000 series tractor.    Ya got  3 speed power shift, and  I normally run in   2nd low,  and  in  intermediate on the  multipower.  .

@ 2400 rpm,    I'm running  5.5-5.6  pulling the  disk,  pulling down 1 gear gets me to  5.0..   I haven't had a  tough spot  yet  that required further shifting.    When it's that tough,  I'll run   1st high and  max out the  powershift to  high...  usually  4.5-4.6

I have to admit,  I'm running  a little hotter  tune than original.    When I bought it  10 years ago,  it was set for  430hp.    I dialed it back  to 360,  and   I have  no  worries  about exhaust temps anymore.   I could routinely  hit  1250-1300..   now I can't get it over 1100. (pre turbo temp)

I usually  set the throttle for about  2400-2450  no load,  and   most of the time it's cruising along  at 2200-2250 under load.

I agree.   The tooling for the  903  was moved to England  a few years ago.. and  its still being  produced for  the military in small  batches. 

That hasn't particularly  helped out with   parts  'here' however.   Something about  keeping the enemy from restoring  abandoned/wrecked  vehicles...   

I used to think that was BS, about  pulling  out of the hole at idle.. until I bought my first one 10 years ago.  I still  scratch my head  on how it's able to do it!

   I still  snicker to myself  when I remember as a kid trying to  get a  JD 8640  rolling  in  high range..  next to impossible w/o killing the engine.  And this was just to road it without  anything attached.

Great engine one with over 21000 and one with 14000 plus. Take care of the antifreeze run the over head max rpm 2300 . They will pull as well as any peek torque is 1800 RPM just have to think different. Guys try to run at 2600 and yes the RPMS will fall until you get close to peek torque. Parts are getting harder to get for both the tractor and engine.

I had one in an over the road Kenworth many years ago, loved the exhaust sound.  It was a strong engine, only problem was with the heads.
Last week I was in a large Cummins dealer shop, was looking at the new engines. Mentioned I had once owned a 9 nothing 3 to the parts man, and asked if parts were still available. He simply said ........................  no.

We had a 4840.  Put 10,000 hours on it.  I would take off at full load from and idle.  I have no idea either how.  Part of it has to be that without the turbo it was probably at it's lowest power setting for the displacement.  265 hp out of that beast wasn't pushing it much.  The 8870 that replaced it requires 1500rpm to handle any load at take off.

not sure what you are talking about.       I deep disk  with  a sunflower 1434 36 foot,   and  my fuel rate is   1 gallon per acre.      What's the problem?

I did not know that. If he used it, that is good enough for me.