Which guns a mercenary company would buy during the early 30's

This would depend a lot on the contacts, resources and preferences of the leadership of this mercenary band.

Surplus arms and arms sorted out as unsuitable or non-standard were often provided on both the official and the black arms markets. Buyers were often Chinese warlords or the beleagrued Spanish republic. Nations willing to support varous ideologically friendly movements or looking to make a buck to re-arm themselves were eager sellers.

When it comes to LMGs, British Lewis and French Chaucats would be available as surplus, as both them and various minor allies were replacing them with more modern arms at the time. German MG08/15 would also be available on the black market - while Germany was not allowed to sell arms, it happened anyway. More modern LMGs could be Czechoslovak ZB-26 (that would later slighly modified become the Bren) or Belgian FN-Browning BAR mle 1930. French FM24/29 could also be available, especially as Greece and Yugoslavia were adopting them as well.

HMGs have German MG08s and Austrian Schwarzlose 1912 available on the black market. Older Hotchkiss mle 1900 are being replaced in many armies at this time, as are older Browning and Colt watercooled MGs. Poland is working to replace their ex-czarist Maxim 1910 MGs and once again FN in Belgium are making improved Browning weapons. Most are still water-cooled for their ability to provide sustained fire. FN-Browning weapons, versions of the M1917A1 are beign adopted by several countries in several different calibers at this time, replacing a mish-mash of British Vickers, Russian Maxims and French Hotchkisses either bought or license produced locally.

I don't quite know what you mean by PM?

When it comes to rifles, there's ex-Japanese Arisakas, bought by the British for the Russians and either held up in Vladivostok or actually delivered to the troops and used at the front and in the civil war (most of which the Soviets would later dump on the hapless Spanish Republic). There's lots of surplus ersatz/replacement rifles, such as modernized German G88s, French Lebels, early marks of the Lee-Enfield produced for the Indian Army and so on. Poland, Chezhoslovakia and Belgium are making licensed versions of the Mauser 98 rifle in various calibers but mainly 7,92 Mauser, and versios of this rifle and carbine can be had on the black market as well. Poland is selling Mosin-Nagants as surplus as well.

There's also multiple smaller arms manufacturers trying to get government contracts or running black market supply to various centres of conflicts, either knowingly or unwittingly with lots of falsified export licenses. As many countries were updating their arsenal during this era, there were lots of hopefuls and small production runs of weapons. Johnson made a competing semi-automatic rifle in .276 to compete for the M1 Garand contract, along with a LMG, both of which were adoped in small numbers by the army of the Dutch East Indees (KNIL) in 1941. Knorr-Bremse in Germany developed a rather bad LMG used by Germany and Sweden, and various arms companies in Europe, owned by German firms through complex schemes saw German engineers work there to develop arms that were banned by the Versailels treaty.