Should my future fiancée be included in the engagement ring purchase. Or should I surprise her with my choice . ?

Buy her a ring that you think she will love. But, be sure you can exchange it if she would prefer another style of the same valve.

Surprise her.       

Include you future Fiancee in the selection of the engagement ring . If you select it on your own if it is not to her satisfaction she will not be happy but will pretend as if she likes it . She will be thrilled when you invite her to do the selection .You are assured of a big hug at the end of the day .

If you don't know her well enough to know the answer to this, perhaps you're proposing too soon.     Prior to agreeing to marry, most couples have extensively discussed it including budget, priorities, preferences, etc. 

That being said, if you were expected to wear something every day for the rest of your life, wouldn't you want a say?   Or at least be asked if you want a say or want to be surprised?

It sounds like you're a bit hung up on all of this being a surprise.   It shouldn't be.   The proposal shouldn't be a surprise either if you've truly discussed marriage enough to determine that you're compatible.    Sure, the exact date, time, place of proposal may be a surprise, but the fact that you've decided you'd like to marry her shouldn't be.   In fact, many couples don't even have a formal proposal anymore.   They simply agree to get married and get on with it.

If the ring is something you want her to love, cherish and be proud to wear in public you should include her in the decision. It's kind of a weird power play to pick something on your own then expect her to (pretend to) love it. 

My daughter and many of her friends have been married in the last 7 years.

Most of the couples had some prior conversation about the rings before being engaged. 

My daughter and her husband looked at rings together online. Discussed prices/budgets . She ended up picking four rings and he picked one ring from those four.

I would have chosen a very different ring from the one my husband bought me. 

She's the one that's going to wear it, so she should be some what included. You should at least ask her, if she has any idea if she knows what she wants in a ring. If she wants a certain type of setting, or type of stone other then a diamond and you don't get it, she may not like the ring that you picked out. 

Everyone is different so it should be a conversation she's part of.  She may want to pick it out with you or she may want to be surprised.

I didn't shop with my husband for my ring but the style of ring didn't matter to me (I'm honest that I wanted one though) and he has good taste and he's not cheap so I trusted his judgment.  He did well.

A lot of my friends picked out the ring with their fiance/husband but not all of them.

She's the one who is going to wear it, hopefully the rest of her life. So yeah she should be involved. 

thats up to you