Hi OP,
Seen as this is your first dishwasher, try not to get too excited over "bling" such as a cutlery draw or metal spray arms.
First is you budget. On MN it's very pay the most to get the best. It's bollocks, cheap can and does last. Our dishwasher is from 1989 and still going, never needed one repair. It's Electrolux, classed as cheap back in the day.
If you do go for expensive, you need top line. No point in buying a Bosch if you are going for their low/mid tier range. It will be cheap plastic shit anyway.
Go for a cheap brand but try to get their high tier range.
Most important is cycle times and how many cycles it has.
You don't want it to be washing for 3 hours just so it can use 1L of water. The wash results will be crap. You want a good wash cycle which uses water and runs for an acceptable time.
Nothing more stupid then having to soak your mugs in bleach because your dishwasher is shit and can't get tea stains out.
Another tip is to run your dishwasher off the hot water, if you can. This will save energy but also help with less wear to the heating element. In the winter water can be freezing coming in so even if your hot water takes a few seconds to come down the pipe it will still help.
eBay also do cheap dishwashers, some are cheap because they have a dent you won't see under the counter. You can also get brand new in perfect condition of course.
Appliances Direct are also good.
As for dishwasher tablets, it's very much like washing powder, you will need to try a few to find one you like. I like fairy platinum.
You will need dishwasher salt but rinse aid is optional, however both are cheap to buy.
Good luck with you shopping! 
