So I'm trying to finish up my first build. All I need is a gas block and flip up iron sights. Using a magpul handguard so I have to use a railed gas block to mount my flip up sight on.  I have a few questions but I'm not exactly sure how to ask/word the questions so bare with me. I want a railed gas block with a slant to the front. Like one of these:
Gas block 1 on dpms Sportacle
This is the link to that gas block: Gas block 1

Or this one that is also a dpms gas block but supposedly "rail height"
http://i58.tinypic.com/2m8012x.jpg
This is the link to that gas block:
Gas block 2

However I'm unsure if I can use flip up iron sights with one or both of these gas blocks without a zero issue.

Looking at MI and Troy flip up iron sights. Any input would be helpful, thanks!
Also is it looked down upon to put an HK style front sight on an AR?

P.s. Thanks to everyone who has responded to my posts throughout my build and tolerated my noobness! You have all helped me tremendously and taught me so much! I'm almost done so maybe soon I'll be one of the ones replying to noob posts lol.

rein:  I suggest you consider the YHM combination gas block and folding front sight @ $102.  It comes in several configurations, it clamps onto the journal on the barrel.  Mine has some blue threadlocker applied to the screws, it has never come loose. - CW

https://yhm.net/accessories/gas-blocks.html?p=2

I use the integrated GB/flip FS from YHM.
However flip front sights are available for rail height or the slightly shorter GB rail.

I almost bought the yhm on but I just don't like the way it looks personally.
How do I know what height to get?

Unless you already have one or the other just get matching heights, it does not matter which unless at some point you think you may want the FS on the main rail.
If you already have either then you will need to get the other in a matching height for what you have.
Make sure the FS you choose will fit on the GB length wise.
I found that out the hard way, fortunately I had an Armalite railed GB that did fit the FS in the parts box.

So you're saying I can make either gas block I posted work?  Not sure how i would zero the iron sights. Seems like they'd be aiming downward no matter what. Unless I got "gas block 2" which seems to be close to the rail height of the upper receiver rail.

I don’t know how to make it any clearer.
The FS and GB must match otherwise you will not be able to zero.
The flip sights are available in both heights likewise the GB’s.

As I look at the picture it appears that the gas block rail is at a lower plane than the receiver rail.  For any sights to calibrate they must be on the same plane OR have sufficient offset to account for the difference.