I would like to invest but I don't own a bitcoin wallet...
I think it would be wise to accept other alts; ltc, eth, dash
You would most likely receive much more funding
You haven't looked at the crowdsale nor read the whitepaper from what I can tell. NVO accepts: Bitcoin, ETH, Ripple, Litecoin, MAID.
You need a Bitcoin address to receive the tokens.
How can be that a problem? When you can easily get a Bitcoin wallet in a matter of minutes. From Coinbase to Xapo you can choose a wide variety of wallets out there. Also I think that they are accepting other coins, look at the first page of the thread you will see that they are accepting LTC, ETH and Ripple for their Crowdsale Process.
No!
Do not invest Bitcoin using any online service/wallet or you will not have access to your tokens. You need a Bitcoin address to which you control the private keys to. This can be easily generated using using Bitaddress or Electrum (there are a lot of other options, but these two are the fastest):
https://www.bitaddress.orghttps://electrum.org/