I don't have numbers on hand for the density of interstellar gas, so I can't speak to the plausibility of using it for the middle part of your journey. However, I can attempt to analyze the other parts of your plan anyway:
First off, tritium is radioactive, with a half-life around twelve years (source). You're not going to want to store it in large tanks- or, indeed, store it at all. Better to use deuterium-deuterium fusion, deuterium-helium3 fusion, or other, heavier stable isotopes; or use the neutrons produced by the fusion reactor to generate tritium on the fly.
In addition, you're not going to be able to use any magnetic field generated by your ship to slow down, unless you're moving into an area where there's actually something to push on, in which case you might be able to use it like a parachute. I wouldn't count on it, though; better to just turn around and use your main engines to slow down once you get halfway there.
Also, rockets don't really have any kind of maximum speed. There's no reason to coast for the middle part of the trip if unless you need to save fuel for later (i.e. to slow down)- and if your Bussard engine is indeed able to collect enough fuel from the interstellar medium, there's no reason to turn it off.
Finally, you're not likely to find large amounts of the rarer isotopes you might use as fusion fuels (deuterium, helium3) in the destination system, so it'd be best to being enough of those for the return trip. Hydrogen and oxygen though? There ought to be plenty. Or, more precisely, by the time this ship could be built, there will be telescopes powerful enough to know what systems have plenty of icy comets and asteroids to mine for water, and you'll be heading toward one of those.
Do note that even hydrogen-hydrogen fuel probably creates extra neutrons, and those could in principle be used to breed more deuterium, tritium, and helium-3. You'll have to look up the numbers on that to see if doing so would be more feasible than just bringing along enough of the rare isotopes for the return trip.
Now, on to your actual questions:
I'd say this design, using a fusion rocket to accelerate to the speeds needed for the Bussard jet to work well, sounds fairly reasonable, if it turns out that fusion rockets work, Bussard ramjets work if and only if they are accelerated to a high speed, and fusion rockets are capable of accelerating your ship to those speeds.
What velocities could be reached? I have no idea. It all depends on the exhaust velocity of your rockets and how much fuel you want to bring along. See here.
What must be taken into account when designing the ship, and what dangers may arise? That's... a very broad question, and I won't even pretend to hit all possible concerns. But you'll need to shield your occupants from cosmic rays, relativistic particles of interstellar dust, space rocks, and radiation from your own engines. You'll need food and water, light, communications equipment, and places for your passengers to sleep and pass their time during the day. And either some way to fix anything that goes wrong, when things go wrong; or enough redundancy to ensure that anything that breaks won't cause any difficulties until you get back to Earth. Or, preferably, both. Because there's always something you'll miss.
Advantages and disadvantages of the hybrid setup over pure fusion or pure Bussard? Well, I've already mentioned the main advantage: if the Bussard jet can only collect enough interstellar hydrogen to bother with if it's traveling close to the speed of light, you'll need some other way to get it up to relativistic speeds. Fusion rockets are potentially one way to do that, and are no less feasible than the Bussard jet itself. After all, the Bussard engine is essentially a fusion drive with the fuel tanks replaced with a giant magnetic scoop.
The disadvantages? It's heavier and more complex, so there are more things that could break. But the Bussard jet should be more compatible with classic fusion drives than with, say, nuclear pulse propulsion- after all, the hard part is getting the hydrogen you've collected to fuse into helium so you can spew the hot plasma out the back of your rocket. Taking a Bussard jet and bolting on a tank or two of hydrogen isotopes for use when the scoop isn't active doesn't seem like a major problem to me. It'll make your ship heavier, simply because you're carrying around an extra tank or two of hydrogen isotopes; but you won't need much more in the way of plumbing or anything like that.