The Spurs don't have all stars in their roster... YET. Sochan, Branham, Vassell very well can become all stars in 2-3 years. Also the Spurs should get good picks in the next few years, whether their own or others (Toronto, Atlanta, Chicago, etc). I sincerely don't think this sense of urgency to contend from day 1 is coming from Wemby himself, but from third parties expectations, and (even if intentions are good) they may be more harmful than helpful.
If you build a team around him and overpay for veterans, the pressure for him to perform is immediate, and that may not bode well for his adaptation process or his body at all. That may also not yield the desired result for all the other youngsters on the team. The no. 1 thing you need to do if you get Wemby is develop him and your team right, and that is more aligned with managing expectations, his own and the rest. Failing to do so also entails a significant cost. You don't burn out your franchise prospect because of external pressure or anxiety.
Like I said, I'd call off the hard tanking phase and try to build a real team organically, probably go for some free agents with upside (Jalen McDaniels?), and if an opportunity comes our way to trade for an interesting player at a position of need (say PG) they sure, I'd entertain it as long as it's in the long term benefit. But if Wemby or his entourage start making the demands implied in your post before he's even steps foot in the NBA, then that spells doom eventually. We've had enough of that to go around the last time we had a star and his entourage try to call the shots.