We don't know where you live. This makes it a little difficult to answer some of your questions about treble clef vs. bass clef. If you live in the UK and will be playing in British style brass bands, then you CAN play tuba in treble clef with the same fingerings as your baritone in treble clef. For the rest of the world, it's best to learn bass clef because all your tuba music will be written in bass clef.
To do this, just bite the bullet, get a fingering chart, and sit down for a couple of weeks and learn to read bass clef and play with those fingerings. Don't try to use any "transposing tricks".
Plastic tubas are crap. Depending on what sort of ensemble you intend to play in and what your aspirations are, it's possible to get a decent, playable, but probably not very pretty, tuba (either Bb or Eb) for a reasonable amount of money. Don't mess with the plastic ones.
Be aware that when playing in bass clef as a "non-transposing instrument", the fingering for a BBb tuba, CC tuba, F tuba, and Eb tuba will all be different.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)