Thanks for your thoughts, it's good to get a different opinion.
On Witherden, he's one of the best young defenders in the comp (check out his Footywire bio page), and should have ongoing natural progression. Would have averaged 87-88 without his injury game, which puts him within 8-9 points of the sixth best defender already. Should take kick ins as well, and is cheap.
On Westhoff, I think it depends how much other value he adds, outside of having a high average. Factors that boost this value include his durability (so he doesn't get replaced by a rookie a few times a year, like a Buddy type), his DPP early on (I can switch he and Xerri to cherry pick scores from Parker or Fort, in some cases having already seen both of their scores), his ongoing ruck cover potential (which also increases due to his durability - there's no cover if your cover also doesn't play) and the structural flexibility he provides in trading (assuming I get Setterfield back to the midfield, I can trade any player in my side to Mumford in round five if I wish, for example, or upgrade anyone to Gawn later on, say). The rough points value of these factors might be 60+40+120+whatever you value DPP trading flexibility at (I would say 100+). Some of those numbers could easily be higher. That's 10-15 points per game across the year. He is priced at 101, so if the test is that you don't want to pay more than he gets you back, then he'd need to score 86-91. I expect him to do that.
In terms of the injury risks, I agree that I'm taking some risks on this front, although there are some mitigating factors in some cases (Bennell and Setterfield have lower JS risk than alternate rookie priced players, eg), and in other cases I'm doing it to access higher scoring potential at a discounted price (basically all the other names). I've also picked quite durable players in Danger, Dusty, Merrett, Neale and Westhoff as a bit of an o***et.
Out of interest, who are the more durable names you prefer?