If the parts are fairly readily available, why does only Watto sell them - especially if he's one of the smaller dealers? As for wanting to help Anakin, Qui-Gon doesn't decide to do that until after Anakin invites them home, which is a long time after Watto first refuses his money - the Jedi could have tried other dealers in the meantime, rather than searching for something to exchange. And for that matter he could have got the parts from someone else (using the mind-trick, if necessary) and still tried to help the Skywalkers.
Look, these are spaceships we're talking about. I seriously doubt that you can just take a part out of one, tinker around with it a bit, then stick it into another, unless the ships are very similar to start with. In any case it's irrelevent, since Watto specifically says that the parts he has are 'Nubian' - i.e. from that type of spaceship.
?? after their ship got damaged they were forced to make a choice about where to land, Tattoine was the closest option.
Tatooine was the best option they had. they had to act quickly.
No one ever actually says anything like this. All they say is that 'there's not enough power to get us to Coruscant', not 'we have to land quickly before the ship crashes' or anything along those lines. No one even suggests that there are any other potentially suitable planets nearby.
"How can you be sure?"
"Its controlled by the Hutts"
"You cant take her Royal Highness there, the Hutts are gangsters!"
so, not only had Panaka never heard of Tatooine, but the Jedi hadnt either. Qui Gon read the Hutt info off the screen, and Obi had to explain about it to Panaka. Panaka had heard of the Hutts, granted, but their reputation was obviosuly fairly far flung, and of course they must have operated on worlds other than Tatooine.
Ah yes, I'd forgotten about that bit. Note, however, that the Jedi are not happy about the Hutts either - Obi-Wan says 'it's risky, but there's no alternative' (my italics). Which suggests that there are no other planets in the vicinity.
And before you say that the other planets are controlled by the Trade Federation, remember you're the one who claims that these planets are too remote to have contact with the Republic. So why would the Trade Federation, which is part of the Republic, have a presence there?
Why should there be standardisation between two areas that have virtually no contact with each other? Imagine that, say, flying cars were developed entirely seperately in both America and Japan - there may well be be similarities between them, but you certainly wouldn't expect to be able to take parts out of one and stick them into the other. And as I said before, it's made clear that they need a specific type of hyperdrive which only Watto sells (or so he claims); if these things were common, Qui-Gon would be unlikely to believe that no other dealers had them. Besides, it's not just a hyperdrive that they need, but other 'Nubian' parts as well - and Watto makes it clear that he has parts for exactly this type of ship (i.e. Nubian).
The thing is that all these things would make a lot more sense if Tatooine was visited fairly regularly by Republic ships, which is what I assumed in the first place. The only problem with this is, of course, that there appears to be no way of exchanging Republic money on Tatooine. I don't know why you keep trying to come up with ever more convoluted and unlikely explanations for this, rather than accepting it for what it obviously is: a gaping plot hole.