I can help you with some of this. First, the 245 is an excellent table. I have a 245 myself, and while there are improvements to be had with the lead-loaded platter of the 255, or even the wood/lead platter of the 265, the 245 is very good sounding. According to Chris at Teres, the lead loaded platter does not make as much difference as the lead loaded hardwood base. It is more of a refinement. You can easily upgrade platters at any time, and Chris has a platter upgrade program which allows your acrylic platter to be lead loaded at about half the cost of a new platter. If you are on a budget, I would recommend getting what you can afford, and using the upgrade path as money permits.
You can switch your RB300 right over onto the Teres. If you want to do this, you can order the Teres 245 with the armboard pre-drilled for a Rega tonearm. I would recommend that you get a VTA adjusting collar from OL or Expressimo at that point, and install it with the tonearm. This will require that you open up the pre-drilled hole by about 1/16" using sandpaper or a half round file. I did it to my armboard in about 5 minutes time. This will allow for accurate VTA adjustment, and is really important for getting the best sound. If you order a OL or Expressimo RB250, then order the VTA collar with it. The OL Silver comes complete with the VTA collar. These tonearms will do well on the 245 and are very wide-ranging in what cartridges they will accept. As far as unipivot tonearms go, they will make a difference, but I think it is a step down. Especially in the lower price ranges. If you decide to get a different tonearm in the future, you can get a new armboard from Teres with the correct hole pre-drilled in it.
As far as set-up goes, you will need an alignment protractor from www.turntablebasics.com and a Shure stylus force scale from the same place. The setting of the stylus force as done by your dealer was a very strange thing to do, and I see no benefit in it. Just set it the way it is supposed to be done.
You can switch your RB300 right over onto the Teres. If you want to do this, you can order the Teres 245 with the armboard pre-drilled for a Rega tonearm. I would recommend that you get a VTA adjusting collar from OL or Expressimo at that point, and install it with the tonearm. This will require that you open up the pre-drilled hole by about 1/16" using sandpaper or a half round file. I did it to my armboard in about 5 minutes time. This will allow for accurate VTA adjustment, and is really important for getting the best sound. If you order a OL or Expressimo RB250, then order the VTA collar with it. The OL Silver comes complete with the VTA collar. These tonearms will do well on the 245 and are very wide-ranging in what cartridges they will accept. As far as unipivot tonearms go, they will make a difference, but I think it is a step down. Especially in the lower price ranges. If you decide to get a different tonearm in the future, you can get a new armboard from Teres with the correct hole pre-drilled in it.
As far as set-up goes, you will need an alignment protractor from www.turntablebasics.com and a Shure stylus force scale from the same place. The setting of the stylus force as done by your dealer was a very strange thing to do, and I see no benefit in it. Just set it the way it is supposed to be done.