Haro MS-4 tires 20 x 2.0 have a great tread pattern that will ride smooth on all surfaces.    They are a good size and if you have been using 20 x 1.95 these tires will work just fine.   65 psi.   The seat and seat post are all one unit.   The size of the post is 25.4 and is 200 mm long.   Most all steel bmx bikes use this size.   The saddle is flat.  It has light padding, I'm not gonna say it is a thick padding but most bmx riders find it enough.   Please visit my store    bike lady     on ebay