Yes, if your mech is just a basic parts exchanger then he is going to charge you a lot, on the other hand if he is into building engines and fabricating, he might not charge you all that much.

Once you do the 3.4 swap, and if it is done correctly(not-Frankenstein Monstered), servicing it would be the same as on the 3.4 Taco or 4-Runner, so any Toyota mechaninic/tech should be able to serivice it w/o problems.

As far as replacing the items you listed, it depends on when they were last replaced, the timing belt is recommended to be replaced every 80k, the water pump used to be every 100k, but now only when it starts to leak. The pullies are probably fine, wont know until the engine is pulled down. If everything else is fine, you could just replace the HG's, you will still need new headbolts(they cannot be re-used), re-using them is one major cause of recurring HG failure. The headbolts on the 3.0 are known as torque-to-yeild bots, they actually stretch durring th einstallation process. Once they are torqued to their desinged specs, you can't ever get a true torque value on them again, so you MUST buy new ones, buy studs, or buy non-TTY headbolts. I believe enginebuilders.com carries non-TTY headbolts for the 3.0. You will also need to buy a new knock sensor and knock sensor wire, the wire gets brittle from the heat, and cannot withstand being disconnected/reconnected.

Figure on paying your mech $1500-2k just to replace the HG's and the nec. items to replace the HG's. If you don't mind paying twice for the the work(sometimes it has to be done), go ahead w/the HG replacement, otherwise II would wait until you can do an entire rebuild, or swap.

For info on the 3.4 swap, check out Off-Road Solutions.