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.