To answer your question right off-the-bat. I'd go with a 914 transaxle first and then some other manual box if you wish. If the Boxster makes you tingle then go for it, but there are some serious mechanical issues to solve for that transplant, not the least of which is the starter placement. More on that from someone who's done it I reckon. As for your other choice, it's an individual thing but I don't like Tiptronics. When I choose to shift I want it right now, not after some undetermined period later. But that's just me and others really like them.
I've run a 914 transaxle with my 3.6L motor and a 2.7 before that for nearly 16 years now and the only thing I've replaced was a synchro while I had it apart to put install an LSD diff a couple of years ago. The synchro wasn't bad but I had another one on the bench that was a little better than that one. I'm plumbed for a cooler (-8AN) as well since the case was out, but it's a simple outlet/inlet (out: the drain plug / in: atop the diff compartment).
I've found that these boxes lubricate just fine without spray bars as long as the pump doesn't flow warm fluid any more than 3 - 5 GPM. With a big-flow pump and the oil at temperature, you would approach dry-sump conditions and that would require spray. Otherwise I think it's unnecessary. The big benefit is simply keeping the oil cool and that'll keep your box in fine shape. That said I have a few rules about our 914 transaxles.
- If you like to spin the tires in first gear then this box may break eventually. 2nd through 5th with the clutch fully engaged you can do pretty much whatever you want.
- Don't speed-shift. A 914 box in good condition will usually do that, but you're wearing out synchros, sliders and spiders, the latter of which are high dollar items unless you have spares around.
- Make sure the clutch is pushed all the way to the floor before shifting. This is true even for slow shifting. See the previous bullet for the consequences
If you really want to take this to it's logical conclusion you have a few options.
1. Don't shift... some folks would be better off with this option but it's a little extreme
2. Shift as little as possible. Most folks shift more than they need to.
3. Double-clutch
4. Match revs both up and downshifting.
Good luck