Lets clear things up on golems and their HD upgrades.
Here it says
Because a construct’s size is limited, a Hit Dice modification cannot increase its size. Therefore Hit Dice modification can never increase the base construct’s Hit Dice beyond 50% of its total HD.
So my golem cant be upgraded to more than 50% of his HD.
Here, under "Constructing a golem"The market price of a golem with more Hit Dice than the typical golem described in each entry is increased by 5,000 gp for each additional Hit Die it possesses beyond the standard for its kind, and increases by an additional 50,000 gp if the golem's size increases.
it says that such golems exist.
Lets get Junk Golem for example.
If i understand correctly, it means that already existing golem can not be upgraded by more than 50% (so 2HD for junk golem, making him 6HD), but at the same time the golem can be created from scratch at 7HD, making him size Large (and then probably upgraded to 9HD, until he must get size increase again). And such golem will cost 5200/4 per additional HD more, so 9100gp + 250gp for scrap.
Now, lets ignore that such golem's market price RAW would be
10200 + 15000 (3HD) + 50000 (size) and could be sold at half price for 37600 to someone who are willing to spend such money on the walking scrap pile.
Another question i have is, if installing a crossbow on the golem as per construct creation rules removes one of his slam attacks. Nothing in the rules indicate so, so i assume he can still smash with the fist if he have crossbow on the forearm (or shoulder), just not on the same turn.