I used to believe all dac's sound the same. back 10 yrs ago, when i bought my first real hifi system I was at a stereo shop in new york and i bought B&W Nautilus 804 and Arcam AVR300 receiver on an impulse mainly because it sounded so much better than what I had before (Kenwood AVR & Polk audio bookshelves).
Came home and hook up my ps2 via optical to the AVR and the sound was certainly better than what I had before but nowhere near as good as i heard when i was in the shop.
I decided to buy a usd 500 denon universal player and connected it via analog AND digital to the arcam avr.
The sound was immensely better (night and day) and it was just slightly better when using the anlog connection of the Denon vs the digital connection. it was easy to test because i could just change the input via remote. There were a lot of different things going on:
1) the arcam in analog mode turned off all digital circuitry and claimed it sounded better in analog (in pure direct mode)
2) the dacs in the arcam were different than the denon (arcam used wolfsen, forgot what denon used).
3) the having seperate powersupplies for pre+amp vs dac+transport
All of those contributed to a much better sound.
Eventually I bought a usd800 Arcam CD player and an hifi friend bought along a Linn Genki CD player along and we hooked all three up via analog to different inputs and played the same CD and kept switching and there were clear differences in the sound. I think the dacs played a big factor, but I think the qualities of the power supplies played an equally big role.
Nowadays I don't have a seperate DAC or disc player. I have a Linn Akurate DSM wihich is a 2ch pre-amp with integrated DAC and all my music is stored in a NAS.
never compared or heard the oppo disc players, but heard good things about them. personally, based on the OP's question, i don't think the forum can ever conclusively answer it.
1) if you belive all dacs are the same (as clearly many here do) then the OP should buy the cheapest huwai disc player and be done with it. why bother even with anything opp?
2) if you actaully want to audition and hear differences, they only you can decide whter oppo 103 or 105 are 'good enough' or whether you need an outboard DAC. the reviews seem to suggest the DACs in an oppo are very very good and a cheap dac will probably at best match it or be worse so you should have some big bux dacs around an you can simultanously hookup the oppo via analog to the amp and via digital via outboard dac to different input in the same pre and input switch via remote which should give you a fairly good basis.
warning though--option 2 requires you to make your own decisions based on what you hear (i know its crazy), and not rely on armchair reviews based on what should work/not work and emprical data on some double blind tests that you were neither and it probably didnt' include the exact equipment in question anyways but some random list of equipment and then the data interpolated to fit gross generalizations on 'all dacs'. it not for everyone expecially double blind abx flat earthers. if you can't trust yourselft you hear differences of an input change wihtout an insanely elaborate double blind abx, then just buy a huawai blue ray and be done with it.
Ignorance is bliss