What is the best way to buy bitcoin for long term investment to sell for cash later? I apologize if this is a duplicate of this question, but it might be because of my ignorance about this topic that I think I am asking for different information.

I would like to buy some bitcoin. About $100 USD worth. From today's rate that is a little more than 0.1 bitcoin. I am fine with hanging on to it for a few years. Maybe five or longer. My concern is that there doesn't seem to be an easy way to sell bitcoin, like through mt.gox. Maybe I'm mistaken about that. I just want to make the purchase and have it to easily sell later, like I do with stock on Scottrade.

Also, if it is on topic, suggestions for litecoin and peercoin would be very much appreciated. It seems like it is on-topic according to some meta posts.
Exchanges work by letting you both buy and sell bitcoin (see How can I withdraw dollars from Mt Gox?). That's who is doing both the buying and selling in most cases: people like you who got their starting bitcoins by buying them.

When you're looking long-term, you should consider the stability and reputation of where you store your bitcoins. MtGox and Coinbase are two options for exchanges you could use, but as the above-linked question states, getting USD out of MtGox isn't straightforward at the moment, while getting it out of Coinbase is (but nobody really knows what the legal status of these exchanges and Bitcoin will be in 5 years). To eliminate the possibility of someone else losing your bitcoins (or you losing access to your account), instead of keeping the bitcoins in the exchange, you could transfer them to a wallet of your own (especially if you decide to buy a more significant amount of bitcoins).

Then the onus is entirely on you: print the private key and keep it somewhere secure (a bank's secure deposit box would be perfect, but too expensive to just store $100 worth of bitcoin), or encrypt it and remember (or write down, and secure the paper) the password while putting the encrypted file on various drives and cloud storages, or whatever else you decide to do. Of course, this means that you might accidentally lose access to your bitcoins if you aren't careful.

You can decide which risk you're more willing to take. I think I'd do the following: Buy it on Coinbase and transfer it to an offline wallet. Secure that as mentioned above, then when you're ready to sell, transfer it back into Coinbase (or whatever exists then) and to your bank account.