I'd definitely isolated test/dev from prod. Physically isolate if at all possible. After all, you should be TRYING to crash your test box, otherwise you won't know if it'll crash in production.
A SAN seems like overkill for less than 100 Gig of data, unless you're planning on major growth in the near-mid term. Couple of 1 Tb drives in RAID 1 would cover that, most likely, and be cheaper. Unless you plan to keep it clustered, in which case a small SAN is useful.
Clustering is good as a first DR step, since it allows for one server to be patched while the other does the work, and so on. Real DR/BC would include clustering for local server issues, and a remote location with log shipping or replication or whatever for things like your power is out for a few days during a hurricane/earthquake/building fire/the janitor is on vacation and nobody else knows how to reset the breaker box, kind of situation.
It really depends on what the business needs and how much they can spend to get it. Everyone wants 100% uptime, very few can/will pay for it.
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