In addition to all the usual parameters of evaluation of a 'regular' broker, I would you suggest you look in particular at the (1) cfd spread of BtcUsd, the (2) leverage for Cryptos, and (3) Swap rates
(1) Regarding spread (crypto cfd's):
If you do not see the spread (bid/ask) listed on their website, as it is the case most often, call an agent and ask it real time and write it down somewhere (ask&buy prices that is). Then call another broker and be amazed to see that the other one is half (or double) of the previous one.
(2) Regarding leverage:
Even if a 1:1 leverage is plenty already to blow your account, you should look for a broker that has higher leverage in case you trade other instruments like forex. Why? Since a crypto trade account is often a separate account from the others, you do not want to 'park' too much of your total account size there. Put enough into that crypto account to trade it, and keep the rest in a 'regular' trading activity account like forex.
(3) Swap rates:
Unlike a forex pair such as EurUsd that have you pay or receive Swap amounts depending if you are long/short, Cryptos don't pay interest and the brokers take good advantage to arbitrary decide on some hefty swap rates if you hold the crypto trade a few days. Ask them what are the Swap rates, very few customers do!!
The fact is that most crypto brokers simply tell you which crypto pairs they offer and the leverage so that they can get customers excited. Other quantitative information that really works in disadvantage of the customer is seldom displayed on the website. Have a full picture before opening a new account. Good luck.