I suggest you do a search of this forum under subject heading "raised beds". There's a couple hundred posts addressing this subject - some very recent - and all your questions (and a few more you haven't thought of yet) will be answered.

Okay. Thanks

When I start a new raised bed in my totally clay area, I start by forking the clay and breaking it up (this can ONLY be done in a 2 week period in the spring! THEN I go to the chiropractor! LOL
Next I find some sort of composted manure in the neighborhood, be it horse, cow, goat, llama, bison....and top the dug up area. I water it and dig it in, then I build a box with a hardware cloth (wire) bottom. I then start adding soil that I get from our landfill (certified organic) and water that into the gaps, then fill to the top.
With the ground dug up under the raised beds, there's plenty of root room! Nancy

You will find something over 146,000 posts about raised beds on Garden Web.
Depth. Plants need a soil depth of about 6 to 8 inches for root growth, so 20 inches will be more than ample. Some carrot varieties might need a little more depth then that 6 to 8 inches but most all vegetables will do fine with that depth.
Base soil. There is no real reason to do anything with the soil under the raised bed unless you have some invasive "weeds" there. The depth of the soil in the raised beds will smother any growth that is there now. You would not even need cardboard or newspaper to put down on that soil. You might consider removing what is in the front yard although with raised beds with a 30 inch depth even that would not be necessary.
Material. There is a lot of misunderstanding about Pressure Treated wood in the garden, as well as heavy metals in soil. The old CCA PT wood was removed from the market because Arsenic could leach from that wood and children could get that Arsenic on their hands as thy played on structures made from that wood and then could ingest that Arsenic. Plants uptake small amounts of Arsenic normally apparently as part of their natural insect protection. Very few plants uptake heavy metals from soils and the most common way people are exposed to heavy metals is when they work the soil. Heavy metals do not move about in soils very well.

My question for people doing raised beds is always, Why do you want them? Accessibility (maybe you can't bend over or are in a wheelchair), soil is very poor, appearance, or some other reason. Many people seem to think they need them to have a good garden, which is not necessarily true. Everything else - the depth, ground prep, etc. stems from that.

As for lumber, the newer treated stuff is made with a type of copper soap which is fairly harmless. Also, I've heard that putting plastic sheeting between the timbers and the soil fill helps them dry faster when the soil is wet (i.e. the wet soil does not sit against the wood constantly). So even treated wood lasts longer. This has the beneficial side effect of blocking chemicals from leaching from wood to soil. You could even use RR ties with confidence if you use the plastic. Although if you're buying, the new treated landscape timbers or lumber is probably all you need. RR ties are heavy. If they were free I would consider it.