I started with a 60 x 30 cm, 60 litre tank. I agree that they are a nice size to start with - though it didn't take long for me to get a 125 litre, then replace that with a 180 litre
The things you need to start are the tank, filter and heater, bottle of dechlorinator (also known as water conditioner), a test kit and a bottle of ammonia. Or a lot of live plants instead of the ammonia. Everything else can be purchased and a fish wish-list drawn up while cycling.
Sand substrate is better than gravel simply because it allows bottom dwellers to be kept. If you don't want those (and a 34 litre tank won't have enough space for them) then gravel is fine.
'Bottled bacteria' products are available; the best ones will speed up the cycle rather than do it instantly as shops will tell you, but the worst ones will do nothing. If you want to use one, Tetra Safe Start is one of the best.
Yes please, find out your water hardness as soon as you can. It should be somewhere on your water company's website. You need a number together with the unit as the words they use can be misleading. If it turns out you have very soft water, there are things you need to be aware of during cycling, but all other hardness won't be a problem.
One final word for now - don't believe anything a shop worker says till you have researched it for yourself. The majority know little about fish keeping and are taught only how to sound convincing and make a sale. They do not believe in fishless cycling!