Basis Debut Gold Standard Mk V or Sota cosmiii

Trane (nice name BTW), I have owned (own) the Illustrious and an SME IV (but not a V). I have heard the V on several occasions, however. Again, these might not be MY first choices but, if you've decided on OL or SME, I'd really suggest the SME IV, as opposed to the V, on the Sota and with that cartridge. I think that you will experience as good, or better, performance and save a few quid in the process. Others can chime in with similar or differing experiences, but most every time I've heard the IV and V in comparison, I've liked the lower extension and midrange of the IV. But, nothing compares to personal listening, so try to audition them yourself if at all possible.

OK

Which tone arm should I go for? (any tone arm for the Cosmos Shelter combination)

...And
"Trane" is for the greatest and wonderful John Coltrane

Yes, the origin of Trane was evident! :-) As to which specific arm, perhaps long time SOTA owners will chime in with their experience but I have HEARD the SME arms on a SOTA to good performance. Perhaps consider a SME IV and a Sumiko Celebration cartidge.

Because you look to be a very new poster here I will offer again the advice that you should go listen for yourself, if at all possible. If you post "what best arm" here you WILL get well intentioned replies. However there may be umpteen DIFFERENT suggestions and, further, those adamantly in favor of one arm will be countered by others who just as adamantly disagree. As an example, I heard a Graham 2.2 on a Cosmos which is owned by a friend and he continues to use this combo, and with a Shelter 901, too. I very much like the performance on that table, though I'd not suggest the Graham for every table. And, I know there are those on this board who do not care for the Graham and would roundly ixnay its use on the Cosmos.

What is the rest of your system – phono stage, amp, speakers, etc.? That help with respect to suggestions, too.

Sorry, no, I only have experience with Top Arms

"Now I need your advice for the tone arm (wick go with the Shelter 901)
On thought is the OL Illustrious Tone arm other is the SME V
Do you have other thoughts on this issue?"

unless you have a housefull of vinyl, don't spend that much. of the two, the sota. in any case, buy used.......